nonono-this is like a legitimate serious thing-I also have a fuckton of war arc headcanons and headcanons on other characters. This comes from years of both consuming media understanding of human psychology.
NONONO YOU DON'T GET IT HANG ON-
This is Dabi's ultimate arc. It starts in war arc and ends somewhere around the end of the show-whiiich according to the director will be maybe a couple more seasons. Dabi fights with Todoroki. And at this point he knows it's his brother, he knows everything. He doesn't care. Todoroki is an enemy. But he's not an archnemesis. Not yet. Dabi doesn't really hate him, outside of him being a hero, and he doesn't really feel any love for him either-he's not capable of that anymore-not in any real capacity. Dabi doesn't have an opinion on him. And then he goes for an attack. And Todoroki's caught offguard-he nearly kills him. But Endeavour saves Todoroki. Endeavour proceeds to fight Dabi while Todoroki goes elsewhere, needing to process his interaction with his father and not really having the time to do it. But Endeavour knows Dabi is Touya. And Dabi knows he knows. The two fight. It is a SUPER charged interaction, with Endeavour basically going "HOW COULD YOU DO THAT TOUYA, HE WAS YOUR BROTHER. WE WERE YOUR FAMILY." No apologies. No questions on how Dabi got there. No concern. Just rage and lectures.
And Dabi SNAPS.
They escape. They retreat back to the PLF, deeply divided and basically having lost. Dabi starts losing it. Memories he's repressed are coming back. He fights internally with himself. He sees constant reflections of his old self in everything and it is TEARING HIM APART. At one point we have a scene of him basically having a panic attack in the bathroom, clutching the sides of a sink and dryheaving. He looks up at the mirror. He sees the white in his roots and screams at his reflection, punching the mirror. He mangles his hand. Never heals properly. gets worse with every fight but he DOESN'T. CARE. He starts losing his grip on himself more and more. His hair goes full on PATCHY. we talking white and black splotches, shaky dye jobs, attempts to cut the white in his hair out only making things worse, guy's gone full on unhinged. It's to the point where he actually starts scaring the remaining LOV members, he's INSANE. This ultimately ends in one final fight between him and Todoroki. Of course, Todoroki knows who he is by this point. At no point during this whole thing has Todoroki felt angry. Just....confused, and sad and overall just-not even really sure how to react. He's been holding back in all of their fights, struggling to get through, relying on help from his friends. But during this fight, he sees the shape Dabi's in and something inside him just sort of snaps. He stops holding back-he uses his right side- He finally figures out how to summon blue fire. Dabi DOES NOT care for that. At this point, he's completely outmatched. He knows. He doesn't care. "All that time...all that training and YOU'RE the one he chooses-it was all for you-I hope you're happy...you're just like him."
But Todoroki's finally evolved beyond this. He knows he's not Endeavour, and he's come to terms with the impact he had on him. Todoroki's moved on. So Todoroki fights him. And Todoroki wins. He ends the fight with Dabi incapacitated, about to get hauled off by the authorities. He looks down at him. "I'm sorry brother...."
That's the last we ever see of Dabi.