There was this manga series, with a bit of a typical storyline. The main character was originally not so strong, but then gains super strength through training (montage!). It's mostly about physical strength though. And this was crazy strong, crazy.

It's not History's Strongest Disciple Kenichi, or the Breaker, but similar.

Any ideas?

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There are times when you will miss what you never had. I wonder how you will find what you so desperately need.

You've got to give more. What kind of fighting? A sport like boxing? Martial arts? Present time? Past? Teenagers? Age progression? Gory? Lighthearted? Etc. Etc.

Martial arts, not boxing. Approximately present time, yeah, and teenagers as well. Not gory, and not sure if it was lighthearted or not. I do thin the main character was a bit on the serious side. More Shin-Woo than Kenichi.

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There are times when you will miss what you never had. I wonder how you will find what you so desperately need.

Baki
Veritas
Karate Shoukoushi Kohinata Minoru
Girls of the Wild's
Kenji
The God of High School
Ultimate Legend - Kang Hae Hyo
Ultra Red
Change Guy

All fit more or less what you said...

None of those I think =/

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There are times when you will miss what you never had. I wonder how you will find what you so desperately need.

Isn't it One-Punch Man?