User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.227.186-20160507145425/@comment-3435913-20160507200336

Issei's evil pieces were stripped from his original body when they tried to summon him through the Dragon Gate, and when he returned they were (re-)inserted into him which reincarnated his as devil again. So he's not half and half, he's a reincarnated devil with his draconic abilities intact.