Regulus Black
Regulus was the first to go, at only 17, and no one, not one person, would discover what he died for for another 20 years.
When he died, even his own brother, who was once his only escape from the hell they were raised in, thought that Regulus was under the command of the most evil wizard to ever exist. But Regulus Arcturus Black was under no one's command, he died for what he believed was right, he died for the same cause his brother was fighting for, of course he did, because Regulus Arcturus Black was not his parents, but he was not his brother either, and he was not willing to let his own parents murder him in the kitchen that day they made him take the dark mark.
Sirius would have, of course he would have, Sirius would have protected him, but Sirius WAS NOT THERE, Sirius had left the year before. Sirius had left him on his own.
And it didn't matter anyway, Regulus died, and Sirius never did find out who his little brother really was.
And so that cold December day in 1979 when Regulus drowned, Regulus put up no fight, because maybe, just maybe, his big brother would get to live.
His last thought is of Sirius, he almost smiled, Sirius had sacrificed his entire childhood to protect Regulus, it was only fair that Regulus would spend his last moments protecting Sirius.
But it didn’t matter, they'll all die in the end.