Denji and Death feel like the two characters in P2 who've had the shit kicked out of them by life. Both of them started out looking relatively fresh, and now they look withered and unwell.
I know most will say it's just the art style change, but I don't think that's the whole truth, as Yoru/Fami/Falling/etc. aren't depicted with wrinkles, eyebags or dry lips all the time.
These two, to me, are both on a downward spiral but we just haven't see Death's take place on-screen.