My character lives on a medieval world, and is a female knight, she is looking for marriage and is trying to look more attractive for potential candidates without losing too much protection.

I think you need to look at this differently.

Marriage, especially among the knighted classes, in medieval times was not primarily about looks (and not even in the West now is it entirely about looks). Social position and property, inheritance prospects and religion or politics had as much, if not more, to do with it. That's not to say that looks were completely irrelevant, but they did take a back seat to many other considerations.

Nor did knights, male or female, wander about all day and night in armor. Only an idiot would want to given it adds a lot of weight and has no social purpose that more comfortable and decorative clothing will not do better. Your female knight will wear whatever suitable clothing and personal decoration are appropriate to a woman of social standing high enough to be a knight. Her social grace, wit, charm and elegance are something she can display in appropriate social settings (just as the men could).

So there is no reason at all for your female knight to worry about her looks in armor. What she'll worry about is staying alive, with all limbs, still able to bear children and on the winning side. Any armor design that does that is what she wants - mobility and protection and vision are the primary requirements : looks are way down the list.

The ability to bear children is absolutely the key role of most women of noble birth in medieval times and frankly for that reason alone it's extremely unlikely her family would allow her to fight at all, certainly prior to her being married and giving birth. This role is of such importance that it brought about the death of more than one queen when they could not fulfill that role satisfactorily, and I do not mean death in childbirth. Without heirs a line may be finished and that would be unthinkable in medieval times.

Most known accounts of women in medieval era war have them wearing armour designed for men. One exception is Joan of Arc who had a suit of plate armour specially built for her. No known images exist from her lifetime, but one depiction (drawn from written accounts) shows the armour as very similar to armour worn by men, albeit slightly smaller and gathered at the waist. The gathering at the waist was very common for men's armour, but probably more exaggerated for Joan's.

In regards to armour that is specifically designed for women's chests, this is a more recent consideration. Some modern body armours are specifically designed for women and while they do not have great drops for cleavage or molded breasts, they do show an actual rise over the chest.

The armour in the picture above was designed to addresses some complaints of female UK Police Officers, as it apparently holds things in place without applying to much pressure.

So while a female knight probably wouldn't have exposed cleavage or molded breasts, it is likely that the bustier of them would have armour that showed a distinct rise over the chest area.

Armor is for protection, not sex appeal.

A live woman is a far better marriage prospect than a dead woman. The whole sexy armor troupe is about serving the male gaze, not doing anything for the woman who is fighting.

Armor is for actual fighting, not walking around.

It's heavy and doesn't breathe and you can't move very well in it. A realistic knight has a squire who's in charge of packing all that stuff and having it ready for battle. Certainly a knight in wartime would walk or ride with some level of protective clothing, but it wouldn't be full armor.

Most women don't have itsy bitsy bodies with enormous boobs.

The stereotype of women with tiny bodies and boobs that made them tip over is, again, created by men for men and is pretty rare in real life.

Real life actors or models that look like that are 1) chosen for those characteristics and are a small minority of women and 2) usually surgically enhanced. Larger breasts are more likely to show up on women who are more voluptuous and/or muscular overall. Like a shotputter. Even then, there is a mix of body types.

Drawings, anime, and 3D representations (like Barbie) that show tiny women with breasts the size of their heads range from almost to completely unrealistic. Most men and women I know roll their eyes at stuff like that. Real life women with natural breasts that huge suffer from horrible back pain and usually have reduction surgery.

Armor is generally custom fitted.

There is not one size and shape of armor for men, so there wouldn't be for women either. Men are different heights, have different torso to leg proportions, different chest sizes, etc. If you don't think most women would be tall enough for armor, consider that teenage boys (who were not done growing) often went to war.

If you can account for these differences, then it's not hard to also account for differences with female shaped bodies too. Even a very large-breasted woman probably doesn't have a larger chest circumference than many male warriors. A smaller waist to hip proportion should also be easy to deal with.

The stereotypical boob armor is not just stupid, it's dangerous.

It should be obvious to everyone that leaving gaping holes in armor to show off skin is counterproductive.

But even when the woman is covered, shaping it for breasts causes no end of problems.

• It’s Time to Retire “Boob Plate” Armor. Because It Would Kill You


• Fantasy Armor and Lady Bits

Attractiveness is in the eye of the beholder.

Fighting well then looking good when she takes off her armor will catch many an eye. Fighting badly (which includes wearing useless or dangerous armor) is a turnoff in all cases. If a potential husband thinks a women shouldn't be a knight, then fighting badly will just reinforce his opinion and make him less inclined to be interested in her. If female knights are normal in that society, or at least if the potential husband is okay with it, then fighting well is an important part of the attraction.

Knowing how to choose and use your gear is essential to being a good fighter.

Besides, even male knights would have a few gems on the armor, head piece, shield, or sword. They might have coats of arms impressed on the shield. Armor might even be painted. So if you want creative clothing, here's your chance.

Taking this in a different tact.

During the time you're referring, the most sexually desirable women had big hips, and a lot of fat on them. Being fat meant you're wealthy enough to eat well and your healthy. http://theconversation.com/womens-idealised-bodies-have-changed-dramatically-over-time-but-are-standards-becoming-more-unattainable-64936

Your lady knight is probably more fit than what is desirable for the time, so a man style breast plate is fine, she needs a way to show off her hips instead.

Also, as a knight, she will be battered, bruised, have terrible skin, and frankly look about as unappealing as possible, both back then and now. Few men were interested in women that did the work of men. They wanted fair looking women with child bearing hips to produce lots of heirs.

So when you really drill down in this, almost none of your original concept works. You would have to first change your world to have female warriors as desirable (vikings maybe?).

Simply having sexy armor would just cause potential suitors that are interested in female knights to think she is wrong in the head.

-Edit
I want to clarify, what was considered beauty was different by place and time. Most nobility values large women but not all. There was a time when women had to tie themselves up in corsets and try to look like boys. This knight lady will need to look beautiful for the time and place she is in. That's why I am suggesting the original concept doesn't work, it needs world building around it to explain why a female knight would be considered attractive.

To answer your other question, how to make this armor viable:

This is actually simple, modular armor. The breast molds have clasps that allow the wearer to attach another plate the joins and fills the valley, creating a man style plate again. She would only put this on before a fight. It would actually provide better protection than traditional armor because you have an entire extra layer of metal. Probably enough to even stop early guns.

Pyramids.

The curved shape of fantasy breast armor would be tricky to make and would not deflect as well as an angled flat surface. A pointed polygon will typically alter the course of incoming energy, deflecting it down to the base of the pyramid where reinforcements will dissipate the force. Sharp points would also allow a close quarters death hug attack.

Regarding attractiveness, that is in the eye of the beholder. Perhaps the pyramids' patterns are attractive.

Push up armor!

She tightly ties her bossom just like she always did and wears the same armor she always did. She just instructs a blacksmith to reinforce and shape the front in a way to resemble attractive, full breasts.

Considering the general lack of fatty foods for all but the wealthiest and the kind of physical work and training she does, her breasts would probably be rather small anyway. You see the same phenomenon in modern athletes.

Real breastplates have a lot of space between it and body to soften blows and prevent situations, where dented plate causes problems. Most females will have no problems to wear a standard medieval armor fitted to her, slight extra space won't make a big difference.

You can just make armor in a style and engraved to show your gender. Good move is to show off your slim waist, so less cheat-days and do not forget space for padding under armor.

If you want to accentuate your assets or they are impressive. You can make extra bulge around the chest. You would want to make it as small as possible, to preserve acceptable center of gravity and general weight and do not look like a joke. Don't over do your back and shoulder pain.

If you want to make separate "mounds" you would need to make it so there still is a good gap between it and your body. Ridge would be a potential weak spot and you would want to make it extra reinforced, same for base of "mounds". I would advise you from using it in battles on horseback, as that is perfect target for a lance or spear.

Can't afford spare pieces of harness - go with a fitted good old one, as your life is more important.

For note, in everyday life you would use minimal amount of armor and for sure it would be rare to go around in full battle armor.

The main problem arising from having breasts, especially large ones, is that their mass tends to move the center of mass and thus altering the equilibrium. Not being a bearer myself I cannot speak about the discomfort of having such a free swinging mass.

This is particularly visible in female athletes, who never ever worry about showcasing their curves while competing. On the opposite, the breast are better hell firmly in place.

A sound design for a female warrior would then require a linen belt around the breast to hold them as flat and firm as possible, and then a formed toracic region to allow for lodging the extra volume. But no protruding things. They would be an easy target in close quarter combat, with likely lethal consequences.

Moreover, denouncing a fighter as a woman with clearly visible breasts can easily hint the enemy for weak points or ad hoc tactics, and of of the principle of war is to hide as much as possible to the enemy.

Regarding instead a parade armor, where showcasing and exhibition is the main purpose, that is not a matter of optimization but about mode and trends.

To make it available we first need to understand why it is not viable in the first place, the reason is simple, it leaves the sternum unprotected while exposing the breasts. Here's a stupid drawing I just made to show a woman wearing boob armor from above.

But there is a way to remove all those flaws, something that actually existed in medieval times called Gambeson

An arming doublet (also called aketon) worn under armour, particularly plate armour of fifteenth- and sixteenth-century Europe, contains arming points for attaching plates. Fifteenth century examples may include goussets sewn into the elbows and armpits to protect the wearer in locations not covered by plate. German gothic armour arming doublets were generally shorter than Italian white armour doublets, which could extend to the upper thigh. In late fifteenth century Italy this also became a civilian fashion. Men who were not knights wore arming doublets, probably because the garment suggested status and chivalry

Gambeson alone was good armor but most of time was also paired with mail and plate armor to absorb impact. By wearing a THICK layer of gambeson under the boob plate all the design flaws are removed and now your knight has full protection.

First, a reframe, then to your problem, then a simple answer, which others have put forth as well.

Boobs are not all that important in the time period you seem to be depicting...
Breasts were not seen as sexually as they are now during Medieval times. Even in the Elizabethan era, take a look at the actual outfits. The breasts are pushed FLAT in most of the portraiture, for a real flat or uni-boob effect, most of the time. Historically, it's not until a little later that boobs get the emphasis...more towards the 1600s and 1700s. But in an era that's all about armor, not so much. The Renaissance does get boob-a-licious--just, seriously look at Medieval to EARLY Renaissance art. And I am talking art created AT THE TIME, not Reniassance or later art that looks back and reframes historical figures in terms of their day's attractiveness. It's not about the boobs. There's a whole different attractiveness standard. Cinched in waist, Exaggerated hips. Boobs are for babies.

But you want boobs because you live in this society and so do your readers, and so neither they nor you can possibly imagine that boobs might not be central to female attractiveness, while still staying true to physics. The answer is not to change the shape of what's worked in armor so that your protagonist can look cute. The answer is in SHADING. This has been used on stage to create boobs where none exist for centuries.

Not form fitting

As any number of people have noted, form fitting armor is stupid and dangerous. Armor is deliberately made with padding between the armor and the wearer. So don't make the armor form fitting.

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  This answer suggests putting the breast shapes outside the real armor. So normal armor with two breast-shaped pieces added.

  
  
  

  I would suggest also making it so that the breast shapes are a softer material. That way, they wouldn't deflect the sword into the sternum but instead pad the blow. So they would give a small amount of additional protection while being slightly clumsy.

  
  


• 
  This answer suggests using paint to give the illusion of shadows to imply different curvature. Note that this might actually make the breast armor better, as it could fool people into striking the armor the wrong way.

Paint it

Instead of painting the armor to give the optical illusion of a different shape, paint the armor to show how the person looks underneath. Paint a face on the helm and cleavage in a dress on the breastplate. Or skip the clothing and paint bare breasts. A lifelike enough painting and gawking attackers might find that they were dead before they realized the lady with breast armor was holding a sword.

It's up to you to figure out whether your character would paint a depiction of her own bare breasts on her armor. Perhaps she is more demure than that. But even so, a depiction of her torso in a dress that happens to highlight her breasts would work.

Multiple outfits.

In just about any society where people wear clothes, people have different outfits for different occasions. The clothes I'd wear on a date aren't the same ones I'd wear to a funeral, a job interview, or to weed the garden. Modern-day soldiers have dress uniforms for looking nice on parade, but that's not what they wear into combat. etc. etc.

Metal armour that provides practical protection is heavy and uncomfortable. It requires padding, which soaks up sweat and gets stinky. It's not something you want to wear 24/7 or even nine-to-five. Your female knight might have:

• One suit of armour designed for serious combat.


• Another suit designed for parades and perhaps for friendly tourneys, which can be as impractical as fashion dictates.


• An assortment of nice clothes for social occasions where armour is not required.


• Clothes for outdoor, riding, wet weather, etc.

Most of her spouse-hunting will be done in the latter. It's not like she's going to be match-making in the heat of battle.

Paint.

Take one regular suit of non-breast-shaped plate, with the chest designed for protection. Now paint it in any colour you like (other than black) and use shading to create the illusion of curvature.

Inflated and spiked chest plates for female warriors who want to show their attributes.

Less about the aesthetics of the armour, and more just how to make it viable as something a woman might wear.

First off, if you’re flat-chested, you needn’t worry about changing the shape of the breastplate. However, if you have larger breasts (which might therefore necessitate changing the shape of the armour), you’re going to have a really nasty time of it as a knight. In order to do any high-impact sports (running, horse-riding, etc.), support is kind of obligatory to avoid shoulder, neck, and back pain - and just mitigate against things moving in the opposite direction from the rest of you.

Breast binding could work (wrap a wide length of linen cloth around your torso, tucking your breasts under your armpits to get yourself as flat as possible), but the method I just mentioned also prevents the expansion of the ribcage, which kind of helps for breathing. People do it, but running around on a field bashing people with swords tends to raise one’s breathing rate a bit, and having a very compressed chest makes life difficult.

The modern option is to use stretch fabric strapping (similar to that used for sports injuries) - you tape down each breast, again tucking under the armpit, but leave a gap at the sternum and the spine where the strapping doesn’t overlap so you still have some room to breathe. Unfortunately, I can’t really think of a medieval alternative to this.

Basically, I’d suggest trying to get your medieval woman to bind as much as she can bear while still having some space to breathe, then stick on some extra boob-shapes lumps to the outside of the armour. Who knows, maybe she could use the space as pockets?

I question the assumption that breast shaped armor would be necessarily be a death trap. Certainly, if a smith beats a sheet of metal into the shape of an idealized woman's body and some girl wears it without much underneath and goes LARPing, she may be more vulnerable than if she wore a T-shirt. However:

• A medieval knight would wear a lot of padding under that armor. This would spread the impact of a blow over her whole torso, so the inevitable hard blows to her sternum would not be felt there.


• Due to the padding, even if the wearer is quite busty, her boobs may not occupy the 'boob space' in her armor, so the armor can be made round on the inside for structural integrity.


• A master smith would reinforce the sternum area, and could make it virtually impenetrable without adding much weight.


• A collar could be placed around the neck area to stop or deflect a spear point from sliding up her armor into her neck.

I would model breast armor on Greek/Roman muscle armor. It was mostly restricted to officers, and it also has the same issue of trapping, rather than deflecting, spear thrusts. Yet it was used. I would argue that the lack of a female equivalent is mostly due to the rarity of female officers at times when muscle armor was fashionable.

Add padding to the outside

Rather than reject the premise of the question, let's assume the character herself does think that adding boobs to her armor will make her more attractive, and this matters a lot to her. Given that, she should probably wear normal armor with a cloth shirt over the outside. Then add extra light padding under the shirt to give the appearance of breasts. This would add a bit of extra weight and make her a little bit less versatile, but both effects would be minor.

I don't think there's a way to make armor-boobs "better" than traditional armor (since otherwise male knights would have worn them in real life), but you can get the effect while sacrificing a minimal amount of utility.