Oh, that could explain why he's so serious in that game o.o
Maybe not "full demon", but Dante could have "embraced his Demon blood/heritage"
I reckon if DMC2 is still canon, DMC5 may be set before it. This game might show us why Dante's personality drastically changed. If he did embrace his demonic heritage, DMC5 may also cover it. I think Dante's age and lifespan would dramatically increase if he embraces his demonic heritage. He'll probably live for millenia and it may be the reason why he looks young again in DMC2.