 If you liked
Night Walker: Mayonaka no Tantei
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 ...then you might like
Devil May Cry
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In both animes the main character is a rather laid-back guy who has to fight monsters as a part of his work. In both animes the main character is accompanied by a girl he has to take care about and a female who comes to help from time to time. In Nightwalker, though, the characters are vampires.