What He Became.

Before being switched for his younger self we see Yamamoto for a few chapters as an adult. Overall this Yamamoto isn't much different from the one we know however there are some differences. Yamamoto's full time job is working a Tsuna's Rain Guardian. The Yamamoto of the future's personality hasn't changed much. He's still a nice guy whose Happy-Go-Lucky attitude encourages and gives the rest of the Vongola strength. The difference between the young Yamamoto and the older one is that the Yamamoto from Ten Years in the future has something darker in his personality. I think this is due to the fact that during the past ten years he's seen a lot, especially dealing with the darker side of the mafia.

Yamamoto just like the rest of the guardians has also lost people in the war against the Millefiore. Yamamoto's father was killed due to his son's relationship to the Vongola. Yamamoto himself has also given up things for the Vongola. He had to give up his dream to play in Major League Baseball in order to follow Tsuna with his sword. Yamamoto still smiles and holds Tsuna and the others close, if not closer to him than he did in the past however it's hard to believe that Yamamoto doesn't have some regrets regarding his choices.

Yamamoto seems to be becoming more distant, he wants to play baseball and has stopped fighting using his sword. This pisses off Squalo because he sees Yamamoto as a challenge to him, it pisses him off so much that he is constantly sending Yamamoto tapes of his challenges to become The Sword Emperor in order to give Yamamoto his fighting spirit back so they can fight more often.