User blog comment:Astarothfan/High School DxD BorN episode 6 review/@comment-3965848-20150510190401/@comment-1359322-20150511055702

A simpler reason, Ishibumi hasn't considered adding Irina into the harem in Volume 6. He didn't even add Irina in the the cosplay party for that reason.

Also probably because Irina reincarnated into an Angel, he made her "pure and innocent" type but changed it in Volume 7.