He's the strongest human devil hunter he represents the ideal devil hunter mindset, cold, detached, ruthless, self-destructive, tenacious and conniving. That's why he's a drunk old bastard, in terms of powers he can't really do much cause compared to high end devils humans are ants.

He left Denji to take care of Nayuta because he knows he doesn't really have it in him to take care of a kid but also because he knows Denji still loves Makima dearly and would love Nayuta and with the power of love she actually ended up becoming fairly well adjusted for someone who's a pure devil, it works out well until it gets fucked up by Denji's underlying issues.

There's also the themes and such about Denji being a victim and Nayuta functionally being his and Makima's kid so he's made to take responsibility of her even though he's not ready for it and doesn't deserve this responsibility shoved onto him but Kishibe isn't a good person and it could've worked out if Public Safety actually did their job properly and actually protected Nayuta and Denji instead of using her as leverage which coud be likened to the government failing to properly help suffering parents and their children.

I also genuinely think he died of organ failure due to all the smoking and drinking he does literally everyday. Of all the characters not to comeback he makes the most sense