Is Pokemon Diamond & Pearl Specials hentai?
No
Rated PG (Children) on MyAnimeList and not tagged as Hentai.

TV Special · 2011 · 2 episodes · PG - Children
Action, Comedy, Fantasy, Kids
One-hour special split into 2 episodes during the airing of Pokemon Best Wishes! featuring the old characters from the Diamond & Pearl series Hikari and Takeshi with their Pokemon.
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