Ok, the bottom line doesn't help, it just shows that he got Endeavor's fire quirk and his mother ice quirk which doesn't support it at all so no, actually, they aren't probably related.
I guess there might be SOME connection like removed cousins or somethin bc remember how it was revealed that Dekus dad can breathe fire soo he might be siblings or cousins with Endeavour .. just a theory
And if ya look at the last sentence, it really doesn't help, just support the fact that Todoroki got his ice quirk from his mother and fire/Hellflame quirk from his father, and if you google who has the fire/Hellflame quirk, it comes up as Endeavor.
