Yo-kai Watch Shadowside - MangaDex

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.

Sly, Yo-kai Watch Wiki

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Welp English manga Shadowside names

Didn't know that Kujira Kaijin in Kamen Rider Black Sun and

Yomi no Tsugai - MangaDex

Uchuu o Kakeru Yodaka - MangaDex

Lightside and Shadowside, Yo-kai Watch Wikia

Yo-kai Watch Shadowside: One JibanMen

SF & Fantasy Manga – 5 Publisher_Shogakukan – Japanese Book Store

Yokai Watch Shadowside by redliger on DeviantArt