User blog comment:B214/Volume 22 spoilers/@comment-71.120.223.31-20161201141018

I also have a theory about Bina, I think she was a member of the old satan faction who witnessed Juggernaught Drive Issei defeat Shalba which peeked her interest and began stalking him until she finally decided to ask him to make her his servant. Ravel objected at first due to Bina's connection to the Kaos Brigade but Issei decides to test her out and give her a chance to prove herself in the Azazel cup.