User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-2402:3A80:D51:78A5:E0EA:3EDE:E483:C5C3-20180301092435

Looks like I may be be able to guess that Issei new trainer is shiva. As it was mentioned to be "samundra manthan" which was war between all Hindu mythology Devils vs lord shiva. Being Hindu I may only guess that