Is Red Garden: Dead Girls hentai?
No
Rated R (17+ (violence & profanity)) on MyAnimeList and not tagged as Hentai.

OVA · 2007 · 1 episodes · R - 17+ (violence & profanity)
Mecha, Mystery
In the future (subsequent to the events of the television series) Kate, Rose, Rachel and Claire divide their immortal lives between study and vigilantism. Their past link to the red garden on Roosevelt Island comes back to haunt them after mysterious transfer students Louise and Edgar seek out their friendship.
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