Osu!! Karate Bu

押忍!!空手部

Osu!! Karate Bu was popular in Japan, but never really caught on in the West. It is still respected and wildly acclaimed as the best 'delinquent shonen manga', and has influenced many other works, and even been parodied by some (Cromartie High School is a loving parody of this particular series).

The story initially focuses on Matsushita Tadashi, but the supporting character Takagi Yoshiyuki ended up becomign much more popular, so the manga's focus eventually shifted to him becoming the lead character.

(Source: AniList.co)