Medaka is literally the last boss tho (also jump MCs are usually the ubderdog who works hard so Zenkichi is the more main character-ish of the two)
Actually the funny thing is shonenn jump series have this bad habit of making main characters NOT the underdogs.
-Goku is just naturally stronger than everyone even before his Saiyan heritage comes into the picture.
-Luffy is stronger than most people, is the son and grandson of two legendary men and has a devil fruit that recently got revealed to be legendary . Also will of D
-Naruto is the son of the hokage, the literal chosen one and as a jinchuriki has incredible powers.
-Ichigo has basically aspects of every special race flowing through his body
-Jotaro has a powerful stand unmatched in speed and power.
-Kenshin is such a powerful swordsman that he was legendary prior to the series.
-Sakuragi is naturally tall and despite being stupid is seemingly gifted at basketball.
I could go on but very few shonen jump protagonists are actually "underdogs". Writers just cant help themselves.