Mutant wa Ningen no Kanojo to Kiss ga Shitai

After a nuclear war, most of humanity was destroyed and the world is now in ruins. Meanwhile, a mutant, born from the nuclear war, meets a human girl and instantly falls in love with her. In this wild land, such an unusual love is immensely heart-rending, and it burns with earnest purity!

(Source: Shueisha, translated)