User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-80.181.161.110-20130914173106/@comment-9339850-20130914173957

the only reason people keep mentioning Tiamat is because she has a name. Theres no way a no named female-dragon/dragon-girl is going to be in the harem after just appearing. And even so they wouldnt join the harem either way after just being introduced, the author likes to give the girls in the harem their own vol where their backstory and progression with Issei goes into detail.