Yuri Koshide

A mild-mannered bespectacled 10-year-old girl who moves into the complex with her parents. She is unexcited to be moving from Tokyo to the countryside but soon befriends her new neighbor Kimi. She eventually feels genuine compassion for Kimi and tries to keep her safe, thinking of Kimi as her little sister.

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In the finale it is revealed that Yuri was acting the entire time and is in fact a fanatical follower of the Cthulhu-esque god named Kuzululu, like her father. She tries to kill Kimi but is easily killed herself when Kimi reveals herself to be a god.

(Source: toonami.fandom.com)