it hasnt been confirmed yet but dabi being a todoroki is a pretty popular theory :)
In the anime the oldest brother Toyo(?) has a red hair which means he inherited endeavour’s fire quirk and he was smaller and skinnier than natsu. Dabi is weak in combat and has a blue fire quirk which might be blue because of the mother’s quirk (ice=blue). He might’ve also dyed his hair black because it’s reminds him of his father (which he hated).
I believe his fire being blue is because it's super hot (remember endeavor's flames turn blue as well when he "turns up the heat").
Also, remember that Endeavor had said that it was a shame that the mysterious eldest brother was strong but had a weak disposition. And that fits Dabi perfectly!
Did they already said what happened to todoroki’s oldest brother? when natsu said that something happened to their oldest brother I suddenly thought that maybe it’s Dabi or something.