She's a Siren. She thought she was the for-real actual Venus, for a while, but Namora recognized her as an evil sea monster and informed her otherwise. It was something of a problem for team cohesion for a little while, but Venus is a basically good kid who's done a lot to atone, and she couldn't really help being evil at the time, it kind of comes with the whole "being a Siren" territory, so they got over it.