I assume he just settles down with Shinobu, which despite the terrible implications works out for the best seeing as she cared for Kira as Kosaku more than she seemed to care about her actual husband. That and because of her we get one of the only lines towards Kira being redeemable where he defends her from stray cat and makes a shitty attempt at rationalizing it as him not wanting to draw attention.

Kira just having a quiet life behind the duwang gang tho ;_;