Tadashi runs into the flaming inferno that was once the SFIT showcase building, but then, PLOT TWIST! He runs back out with a very startled Callaghan slung over his shoulders, moments before the structure explodes.
After the incident, Tadashi thinks, “Someone always has to help…and that person should be me.” On top of that, he gets it into his head that he’s capable of running into flaming buildings and coming back out unscathed. He starts training to be a firefighter (he luckily already has paramedic training), and when he graduates SFIT two years after the incident, immediately joins the San Fransokyo Fire Department.
He quickly earns himself the status of greatest risk taker on the force. He’s fit and intelligent though, and succeeds in saving many lives. But each success, however, leads to him making increasingly risky decisions.
Every day, Tadashi returns home to be patched up by Baymax and 16-year old Hiro (currently a grad student at SFIT). Hiro knows of his brother’s behavior, and is terrified that every day will be his last, but Tadashi waves off his concerns, claiming that “Someone always needs help, and sometimes I’m the only one who can give it to them.”
After a close shave, Hiro finally has enough. Enlisting the help of Fred, he creates a masked hero identity. He hacks into the fire department coms and conveniently appears every time Tadashi’s team heads into danger. People start calling him “Hero.” Tadashi quickly catches on to the pattern that Hero only appears when he’s in danger, but instead of stepping back and acknowledging the help as Hiro had hoped, decides that it’s his personal mission to be the one to save the Hero who saves everyone else.
Extended headcanon below, along with optional Hidashi plot (which will be very, very obviously marked as Hidashi)