Sending hugs to you, Moko. How are you doing? Have you looked at the Lessons?
https://bpdfamily.com/message_board/index.php?topic=56206.0 The Lessons are a good start to understand how BPD might be influencing your relationship and what you can do to make things easier.
Your question "who is this person?" makes a lot of sense because one of the defining characteristics of BPD is no sense of self and a negative or unstable self-image. You don't know who this person is because he doesn't either.
It's an unpopular thing to say these days, but I think that gender dysphoria often arises out of another mental illness or social factors that make people deeply unhappy with who they are. I'm glad he's seeing somebody who's willing to look into that instead of pushing him to transition. Is he planning to transition?
Remember that you don't have to be or do anything that doesn't feel right to you! If you don't want to be married to a woman (or a man in a dress), you don't have to. You'll probably experience a lot of pressure to "accept" him and "support" him through HIS difficult time, but this is a difficult time for you too and it's okay to struggle. You don't need to let yourself be forced into a "lesbian" relationship if he transitions. He shouldn't get to decide to change a core part of YOUR identity.