i'm searching for a manga where the main characters at the beginning is weak ( better if he or she is the weakest) and throughout the series he becomes strong only thanks to hard work and effort (not shoujo)

shamo but read the comments before reading it.

um...the first thing that came to mind was History's Strongest Disciple Kenichi

Aria, technically speaking. As long as Akari isn't rowing backwards.

Probably not what you meant though, as it's not exactly a traditional shounen storyline. Sorry, I suppose this was a bit of a misleading suggestion, but I felt throwing it in there was necessary for reasons beyond my own understanding.

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"breaker", its been 42 chapters and hez lost most of his fights ahaha

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it's a midway between shamo and kenichi because in kenichi there is too comedy and sometimes is unreal( the grandpa is almost superman) while shamo is too dark with the main characters that is really bad and has no esitation to kill. I want a good characters with more serious aspect... the breaker is a good example even if the main characters is unbeatable and the student gains power thanks to a medicine