User blog comment:B214/Question and Answers/@comment-162.202.198.72-20130829234142/@comment-9339850-20130829234545

main reason why serafall isnt in the harem.. She's a siscon. MAYBE, in the extremely off chance that Sona becomes attracted to Issei and might want to be in his harem, only then she MIGHT consider joining the harem as well to stay close to Sona