Since a lot of my canons want to be pains, yet again I have to do a 'what is canon for me' post. *headdesk*
Okay, he and Aya-chan end up in the hospital as per the anime series. He joins Crashers as per manga and leaves as per manga when their 'no killing' interferes with his revenge, besides he was just helping out anyway so he figured it would temporary. However, his initial meeting with the rest of Weiss goes as it does in the managa: He and Ken don't get along and fight. Aya isn't quite the loser, but he is beat up enough. He doesn't have a room yet so he's put in Yohji's room. He won't leave so they share a room that first night. Upon hearing him talk in his sleep Yohji hears the name Aya and believes it belongs to the young man, or is important to him. Either way that's what Yohji calls him. Aya had already made this decision, though he used Ran in Crashers that was mainly because he went mostly by his codename, Rook, and the fact that he didn't intend to stay with them, he doesn't correct Yohji.