User blog comment:B214/General Discussion/@comment-1359322-20161024045747

https://www.reddit.com/r/HighschoolDxD/comments/58sg46/summary_from_the_dxd_session_at_the_fantasia/

This is quite interesting. So


 * 1) Rias, Akeno and Koneko originally weren't planned, they got added due to change in plot. Thank god for that.
 * 2) Rias was originally supposed to have smaller breasts, Ishibumi changed his opinion after seeing Rias' illustration. Thank you Miyama for being the one illustrating Rias and Ishibumi for accepting it.
 * 3) Issei's original character supposed to be Kiba, that means Kiba wouldn't exist then. Thankfully that didn't happen.

So in the end, aside from Issei and Asia, the remainder of the big six originally weren't part of the story.