User blog comment:B214/General Discussion/@comment-9595671-20161013202130

Oh boy, I think I realized why Ophis seems to not really be focused into being part of Issei's harem (unless it's planned and hasn't happened yet ^^;) despite what her trivia says... she sees Issei as her daddy.

That's right, at this point of the story, Issei may have well unintentionally adopted Ophis. I mean she's called him "Papa" once if I've got my info right, listens to him like a child would listen to a parent, and she is mentally young enough due to her naivete to fit being a child.

With the fact she's also called Rias and Akeno "Momma" prior, I guess that means Rias already won in the department of having a child with Issei... indirectly that is ^^; (I wonder if Ex calls her "Big Sis" if she doesn't reach harem status?)