Bishonen 【美少年】 refers to "beautiful young men", or "prettyboys", a common style of character design in Japanese media.
Especially in more recent games, power for male characters is inversely related to how effeminate they are1. This is doubly true for evil characters, and in modern games, you will very rarely find a musclebound evil character who isn't a useless mid-boss. The advent (pardon the pun) of this trend in video games is usually attributed to Final Fantasy 7's Sephiroth. Castlevania: Symphony of the Night's Alucard was another notable bishonen contemporary.
1 There are some minor exceptions to this rule, like, if you're Pierre Mazywell from Shadow Hearts II, a tailor who exchanges homosexual trading cards for little girls' dresses. While I haven't played the game myself, I'm quite confident he's not terribly powerful