The Stray Cat's Cheat

Private detective Ousuke found an unexpected guest on his doorstep one day: his old friend Yaichi, now an ex-yakuza with a strange new power... and a mission for revenge. He was looking for the man who killed his beloved mentor, and Ousuke agreed to help him. He never was able to resist rescuing the guy. Even worse, almost as if he knows Ousuke is pining for him, Yaichi crawls into his bed whenever he has nightmares of his mentor's death, tormenting Ousuke without even realizing it. All Ousuke wants is to get away from Yaichi once and for all before his old feelings take over again, but that might be harder than it sounds...

(Source: MangaPlaza)