Isekai Nippon: Ansatsu Ikka no Sannan wa Kotokaika shita Nippon de Musou suru

When two worlds collide, and a fantastical other-world is combined with modern Japan, a fateful encounter will occur! A boy known as a "talented killer" will meet an "isekai princess" in this story of boy-meets-girl!

(Source: Cycomics, translated)