Ecchi series' that are borderline Hentai

Kampfer is a series that's a bit riske.

Its a story bout a guy who turns into girl, but its not Rama, its Kamper, its German! And theirs a bad guys as usually but I can't tell, because theirs so man girls who want want the guy, and the guy girl too, and that's pretty funny, so lets lets make it funnier, its Kamper Abridged! DONK! Okay give me my money.....

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Koe de Oshigoto has two episodes plus a pretty nice manga.
The author also has done several actual hentais as well if you like his art style.

Yosuga no Sora is the first one that pops up in my head.

Rosario+Vampire... nuff said.