I also thought that it might not be the same person who wrote that to Masamune. But since it's almost impossible to get that nickname by chance/accident, it has to be someone he knew when he was was and who knew about the nickname.

As for revenge... Well for the moment he's only having or wanting a revenge towards himself at the time, I mean the fatso one.
And somehow, it's possible the girl had interrest in the fatso...
Since I'm pretty sure today's Masamune isn't very different from fatso Masamune, his love for the girl should overcome his cheap revenge should the girl fall in love with him.