Zenra Otoko Series (Light Novel)

Newly-instated detective Ishida Tomohiro returns home one day to find a naked man in his house. He asks Tomohiro to identify himself, only to vanish into thin air before the startled young man can come up with a response. Soon afterwards, Tomohiro becomes tangled up in an odd situation involving a killer sword that can possess people. Then the same guy shows up again: this time fully clothed and actually physically present. "Kyousuke" is a member of the supernatural task force affiliated with the police, and Tomohiro ends up being transferred there. Now he'll have to handle all sorts of bizarre and out-of-this-world situations alongside his extremely quirky new colleagues!