 If you liked
ChäoS;HEAd
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 ...then you might like
IS: Infinite Stratos
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They're both harems, to say the very least. You have the main hero guy getting more girls one-by-one as the story progresses. Interesting to say, these are both action series where the guy isn't just standing by the sidelines while the ladies do the work. They take the effort to be useful and stand up as a hero to them.