Yuuki was missing the company of male buddies of the same age. That's why he was so happy to spend time with Kei as well, before he rejoined the school life. In such company he can behave differently compared to work or when he's with his girlfriend. Being deprived of such companionship would also strenghten his maidenly side, and vice versa. Mayu just needs to get used to the truth you can never know another person fully.