Shadowverse: Rise of Bahamut

If this is a male character, why is he wearing a breastplate?

Well, strictly speaking, a breastplate can suit a guy too; not everyone has to wear full plate armor. Especially if you're climbing trees. The boots, the feminine eyebrows a-la kitten, the thin arms... those raise more questions.
However, if SV doesn't create the artwork themselves that they insert into the game (based on other artwork, I have the impression that this is the case), then everything is explained very simply: they themselves didn't understand. If not... well, then it's just the peculiarities of Japanese artists.