Nissa's ending can be reached from Sirath's route. You would need to romance Sirath, then change your mind at one point when she asks you.
Oh so to date the girl I've got to start with a guy😞? How long would I need to do that?
I don't remember how long exactly. She doesn't have a full route, only a different ending, so unfortunately that's how it ended up.
OK thank you for your time and letting me know.
