The popularity's interesting because he's objectively not designed to be a classic heartthrob, right? That's kind of the point, I think. The appeal sits in the gap between his canonical presentation—scarred, angry, consumed by revenge—and the fanon reinterpretation that often paints him as deeply wounded, tragically romantic, and needing 'fixing' or understanding. It's a classic 'hurt/comfort' and 'beauty and the beast' dynamic bundled into one character.
A lot of stories lean into the 'untapped potential' angle, too. He's a Todoroki, which ties him to a massively popular family dynamic full of drama. Exploring what a relationship could do to that vendetta, or how someone might see past the literal burn marks to the person he was before, provides endless fuel for angst and redemption arcs. It's less about conventional attractiveness and more about the intense, dark emotional aesthetic he represents.
Plus, his design, with the staples and blue fire, is just visually striking. That translates to a powerful, distinctive vibe in fanworks, which writers and artists love to play with. He's a character defined by extremes, and that naturally fuels extreme, passionate shipping.