 If you liked
Trinity Blood
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 ...then you might like
Vampire Juujikai
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Both are great mangas with the vampire theme related to sci-fi (vampires came from space). Both have outstanding action scenes and a awesome plot, where the main guy is a very old vampire that has huge terrifying powers but hides them and hides his painful past behind a smile and a gentle clumsy personality. He also wants to protect humans and vampires no matter what and tries to make them coexist in peace. Also, the woman he deeply loved died in the past.