 If you liked
Yu☆Gi☆Oh!
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 ...then you might like
Meitantei Conan
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Yu-Gi-Oh and Detective Conan are two iconic shounen manga where the main character is a boy with some kind of alter ego who demonstrates his smarts and problem-solving as he punishes the criminals who comes in his way (for YGO this is mainly in first 7 vols). The girl he likes is trying to find out who this alter ego is and the series has some elements of mystery, though these apply more to Conan than to YGO. Both series are really funny and engaging to read, but also really long (Conan is STILL serializing as of 2021!) so keep that in mind.