Youkai Watch: Shadow Side
妖怪ウォッチシャドウサイド · ようかい うぉっち しゃどう さいど
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
Taking place after the events of the fourth film, the series centers on Keisuke Amano, Natsume's brother who doesn't believe in supernatural phenomena. But as he notices his sister is frequently comes back home late, he decided to stalk her, only to find out she is exorcising evil Yo-kai and running a Yo-kai detective agency. With their secret revealed to him, Natsume decided to let him tag along in her activities as they investigate Yo-kai crimes and take on evil Yo-kai to maintain peace all over the city.
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections