It was a 3D game, and I'm fairly certain it was just a side character, and she could transform into a normal looking human form if she wanted. During her sex revisit scenes you could choose between fucking her real form and her human form. I'm pretty sure she had like slight buck teeth too? Maybe even a little bit bunny like in appearance? This is really all I have to go on. I just remember her alien/monster form being pretty tall, very thin, maybe even bony, and she talked about how her alternate form was normally not enticing to people (ofc).