A trap/femboy is a guy that looks feminine and can pass for a girl in a lot of instances aka: Berwyn, Quin, Nikol, Nakano and so on.waits whats the difference?
An ambush is a more swole/muscular guy that still sways towards the androgynous aide of things -- the closest one in game being bust Elyon and Rooh.
For more out of game examples are Beelzebub from Grand Blue fantasy, Zato and Axl from Guilty Gear and Diavolo from Jojo. It all mostly comes down to a cute face and hair free skin.
Tl;dr: Ambush is just a muscular femboy