Joss wasn't nearly as goofy as Keith was. At least she could appreciate his goofiness. Most people couldn't.
He always had to tone his personality down depending on who he was interacting with. But there was something about her that made him feel comfortable enough to be himself. Maybe it was because of her psychology training.
Most of his life was spent acting. Acting how he thought the foster families wanted him to act so they wouldn't send him back. Acting how he thought the kids at school wanted him to so they wouldn't ostracize him. Acting like a normal person who did not double as a hero. Acting more heroic than he truly was.
He didn't act around Joss as much as he did around the others. He couldn't tell her about being Delta but, aside from that, she got the most authentic version of Keith there was.