 If you liked
Zero no Tsukaima
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 ...then you might like
To LOVE-Ru Darkness
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a boy is summoned into a fantasy world to be the familiar (magical servent) of a teenage girl. like to love-ru there is plenty of echi and comedy as well as a harem of sorts, but their is more action, storyline, and romance.
Both the light novel and its anime counter-part are great in their own way and i would definatly suggest it for any fans of To love-ru