One of my favorite Todoroki OC backstory ideas revolves around a forgotten sibling, someone who was hidden away due to their quirk being deemed 'too dangerous' by Endeavor. Imagine a child born with a quirk that merges fire and ice uncontrollably, creating chaotic explosions. They’d be sent to a remote facility, only to escape years later, harboring resentment toward both Shoto and their father. Their journey could focus on reconciliation or vengeance, depending on how dark you want to go. The emotional tension between them and Shoto would be intense—both victims of Endeavor’s ambition, but with wildly different paths.
Another angle could explore a cousin from Rei’s side of the family, someone with a pure ice quirk but raised in a loving environment. They’d serve as a foil to Shoto, showing what his life could’ve been like without the pressure. Their dynamic could be bittersweet, with the cousin trying to heal Shoto’s wounds while grappling with their own guilt for having a 'better' life. This could dive into themes of envy, forgiveness, and the weight of legacy.