Sirius Black
History changed and future was made.
Harry Potter didn't stop crying when Hagrid smiled at him. Instead, Light Yagami cried harder.
Sirius Black didn't meet Hagrid when he was leaving. Instead, he met the half giant in the midst of the rubble of his best friend's house.
Harry Potter, in some alternate universe, had already fallen asleep by the time Sirius Black came around. Instead, Light Yagami was wide awake and only stopped crying when this strange man who reminded him of his once best friend, Ryuuzaki, held him.
Sirius Black didn't go after Peter Pettigrew alone and in grief, thinking himself unnecessary in Harry's life. This time, he knew Harry needed him in his life and that going after Peter was not as much about vengeance as it was about protecting his godson.
McGonagall might not turn up to help them track down Peter Pettigrew, but she spread the word. Remus Lupin, who did turn up, tracks down Peter Pettigrew along with Sirius Black and explains to the Aurors that "No, Sirius did not blow up the street. It was Peter Pettigrew. No, Sirius was not the Potter's secret keeper. That was Peter Pettigrew too. No, you can't convict Sirius just because he's hysterical in grief. Yes, we demand a trial and yes, we accept Veritaserum."
In some universe, Sirius Black was alone and stricken when the Aurors and Obliviators track him down. He goes to Azkaban without a trial and without anyone on his side.
This universe isn't it.