User blog comment:B214/Volume 22 spoilers/@comment-26345952-20160722133217/@comment-26345952-20160722135419

Maybe, Tannin is a High/Ultimate class devil. The question is, was Bova's egg laid before or after Tannin became a devil?

As far as I can understand from spoilers he wants to join Issei, but not as his peerage, though Issei might consider him in the future.