User blog comment:B214/Question and Answers/@comment-82.178.40.107-20140127122853/@comment-16281343-20140127123640

Why wouldn't they?

Even if Issei is a reincarnated Devil, Issei's existence is one of a kind. He houses Ddraig, a Heavenly Dragon and his body is created from Great Red's flesh using Ophis' power. The names mentioned are considered heavyweights in the DxD Universe :)