As I remember it was a comedy series on US tv, English language, probably Nickelodeon or Cartoon Network.
The protagonist was a teen sidekick of a superhero, balancing his normal life with sidekicking. But his superhero, named Mega Man, Megaman, Maxi Man, or Maximan, was missing.
Thus the opening theme song began "Megaman is missing..."
The series is about an orphan boy named Eric, with his best friend Trevor and his two female friends, Vana and Kitty, who are training to become superhero sidekicks at the Academy for Aspiring Sidekicks (though in "Graduation Daze", supervillains can send their henchmen to the academy) in the Canadian city of Splittsboro (based on Scarborough, Toronto). While dealing with intense sidekick training, Eric must also contend with his strict guardian Maxum Brain, his grumpy teacher Professor Pamplemoose, the evil Master XOX and keeping secret the disappearance of his superhero mentor Maxum Man from the city!
I don't have a great story for how I found it. I searched for tvtropes cartoon sidekick to find cartoon shows about sidekicks, and I saw that it had a "Maxum Man" who was missing, and then the theme song matched.
