User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-31764731-20180326095200/@comment-30924344-20180326135008

His soul was put into a vessel made from Great Red meaning, his flesh and blood are that of a dragon's. Issei is a humanoid dragon like Ophis. With the addition of having Ophis' power, it's very possible for him to become one.