Advice?

So there’s this guy that I’ve always kind of had a crush on, but it was like an “If life was perfect” crush. I never would have imagined anything actually happening between us. But now he likes me and I like him. But he has a psycho ex girlfriend and because of her I don’t want anyone knowing about us. Because she’s the type that would target me and either mentally hurt me or physically hurt me. I’ve never been scared of anyone until her. But I feel bad for her because I can tell she depressed and lonely, so I’m there for her when she really needs me. But I don’t know what to do. Any advice?