User blog comment:Crimson and Darkness/Shin DxD 4 discussion/@comment-40183495-20200221101007/@comment-190.46.93.180-20200221142153

the growth of issei is a miracle, going from a lower class to a top 10 demon in a year is something that neither training justifies, it is a miracle or eternal support from other characters (such as the power of ophi or the drug of indra) what they have led the enormous growth of issei ... had not seen such growth since ichigo or protagonist isekai.