User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170206224540/@comment-31137559-20170207012731

Well Tiamat is the only Dragon King that Issei hasn't met and she's female and labeled as the 'strongest Dragon King' so they could have been how they met because she's a dragon and was intrigued by Issei's unique growth so she decided to see if Issei was worthy of having her as a servant.