User blog comment:B214/Volume 23 discussion blog/@comment-76.175.162.45-20170402000902/@comment-30924344-20170402001818

Nobody thought Elmenhilde would join the harem; a woman who was such an arrogant bitch at first so Tiamat joining who's not a bitch [just a woman with anger issues thanks to Ddraig], is more plausible then Elmenhilde. Tiamat would join thanks to Issei most likely finding and giving her back the treasure she's been looking for. Also Dragons are natually interested in strong people the most, so that is a reason also, but in the case of Tiamat, Issei returning her treasure would make him look like a "strong man" in her eyes; in this context: a man able to perform a specified action well.