User blog comment:B214/Talk About What You Want! 2/@comment-4547267-20140703192841/@comment-1359322-20140704003615

Before you consider Kuroka to be a suitable Bishop for Issei, does Kuroka even want to become one. Like Kuroka said before, she's a stray cat, she doesn't want to become anyone's servant/Evil Piece anymore. For Kuroka being in the Vali Team, suits her more than being a member of a person's peerage, even if it's Issei.