YOUKAI WATCH: SHADOW SIDE

Keisuke is a fifth grader who does not believe in ghosts, UFOs, or even youkai. Lately, he has noticed that Natsume has been coming home late, and one day decides to tail her. When he discovers that Natsume goes to a youkai detective agency, Natsume is forced to involve Keisuke in her activities. (Source: Anime News Network) — Description from AniList community (anilist.co).