User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-14.201.211.143-20150720115807/@comment-3477384-20150720120231

Great Red's contribution was Issei's body. Without the body made from  Great Red's flesh, Issei wouldn't have been abel to wield Ophis's power, he would have been crushed under the pressure of it.