Is Uchuu Kyoudai: Number Zero hentai?
English title: Space Brothers #0
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

Movie · 2014 · 1 episodes · PG-13 - Teens 13 or older
Adult Cast, Comedy, Seinen, Space
Prequel to the TV anime series revolving around the "origin of the dream."
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