User blog comment:B214/Question and Answers 4/@comment-24331706-20150319150433/@comment-26216310-20150320002647

Well, I only followed the manga and the anime at the moment so my knowledge isn't as vast as those tht read the light novels on this show, but, if you did watch the anime, in the last fight scene between issei and vali, issei absorbs some of the power of the white dragon and infuses it into his right hand. The white dragon scolded at him and said that he (issei) had a long life ahead (as a devil he ends up becoming immortal would be my guess), but due to issei absorbing part of the white dragon he ended up "decreasing his lifespan considerably". So my take on it is that yes issei did become immortal but after he absorbed a part of the white dragon he reduced his lifespan. I could be wrong ofc so I'm wrong please anyone correct me. I hope that helped you out fellow HS DxDer :D