 If you liked
Rosario to Vampire
|
 ...then you might like
Shijou Saikyou no Deshi Kenichi
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Both stories are about a weak boy who finds himself being protected by a strong girl, so he decides to train/improve himself so he can be the one protecting her instead. Along the way, it turns into a harem story and a childhood friend (who practically looks the same in both stories) returns as the ultimate rival.