On Kayle's Armour (and Female Armour Generally)
The above video by Shadiversity, a YouTube channel which explores medieval combat and it's applications in fantasy, addresses a lot of the concerns which people have with feminine armour overall; but for those who'd prefer to read, here's a summary:
- Armour with feminine designs weren't made historically, not because it wouldn't be functional, but because next-to-no women fought in war during this period.
- IF female warriors had been more common, it's entirely feasible that armour made to accentuate a feminine form would have been made, as many traits of male anatomy were present in historically accurate armour. These included different fashionable chest-to-waist ratios, pectoral muscles on the breastplate (men had boob-plate), and codpieces (which were basically big metal penises for your armour).
- The design of larger mounds on the breast-plate will not have enough of a detrimental effect on the armour's ability to deflect strikes to discount the design; at least, no more so than contours typical, male armour.
- Armour-bikinis are a stupid idea though, he has a separate video on how to get the "Barbarian" look with better functionality.
As for 
Kayle's armour specifically, the main issue isn't the feminine design of the armour, but the distinct LACK of armour on her abdomen. However, this can easily be explained away with the assumption that the cloth section his a type of fantasy, skin-tight gambeson. While not AS effective as plate, gambeson is an effective armour option if you're looking to trim down the weight of your armour, about as effective as leather armour overall. Gambeson is Shad's recommended armour of choice for the typical "Adventurer" character, but here it can be argued that Kayle's trying to make it easier to fly.
Now does this mean that all female armour sets are made with functionality in mind and not just to use sexual appeal to market the character? Of course not. Gotta make the money to keep papa Tencent happy. However, I've seen a lot of people look down on feminine armour designs out of a misguided view on functionality, so here we are.
