His Reason For Fighting

Yamamoto is the second to join Tsuna's family, after Gokudera. He either knows Tsuna's guardians prior to the start of the series, or he gets to know them as Tsuna does. Unlike most of the guardians who distrust or outright dislike at least one of their fellow guardians, Yamamoto is friendly with each of the guardians. Even if they don't feel the same they all have at least a begruding respect for him.

Sawada Tsunayoshi

Yamamoto and Tsuna's friendship began when Yamamoto showed Tsuna kindness by letting him join his team in gym class. Tsuna repaid that kindness by talking Yamamoto off a literal ledge and then saving Yamamoto when he nearly fell trying to get down. From that point on Yamamoto became Tsuna's friend and was eager when Reborn invited him to join the "mafia game". Yamamoto is loyal to Tsuna and is willing to sacrifice himself if it means keeping Tsuna and the others safe. When he is unable to defeat an enemy he worries and pushes himself until he becomes stronger because he's afraid of letting Tsuna down. Tsuna appreciates Yamamoto's commitment but he often seems concerned that he is involving Yamamoto in a world where he doesn't belong.

Gokudera Hayato

Gokudera and Yamamoto have a tumultous relationship. When they first meet Gokudera immediately sees Yamamoto as a rival when it comes to fulfilling the position of Tsuna's right hand man. Yamamoto doesn't take Gokudera's threats seriously but will often egg him on jokingly by claiming he'll become Tsuna's right hand man while Gokudera can be Tsuna's earlobe. Despite their constant taunts and insults the two of them become good friends because they will both put Tsuna first and do what needs to be done. They work well together and Gokudera's ranged dynamite attacks compliment Yamamoto's close range swordsmanship. Yamamoto encourages Gokudera to be a better right hand man by chastising him and telling him that he is unworthy so long as he turns away from his fellow guardians instead of uniting them.

The Rest of the Guardians

Yamamoto is universally respected or liked by the other guardians. Sasagawa Ryohei respects Yamamoto's abilities as an athlete and his willingness to participate in whatever "extreme" activity he has planned. Lambo Bovino thinks Yamamoto is much like the rest of Tsuna's friends, fun to play with but overall a servant meant to be subjugated by "The Great Lambo". Hibari Kyouya often threatens Yamamoto for being part of Tsuna's trouble causing group but when it comes down to it he respects Yamamoto's immense skill with a sword. Even when Gokudera and Tsuna are concerned that Yamamoto might not pull through in a battle Hibari is quick to remind them that "He is Yamamoto Takeshi." meaning that Yamamoto won't fall easily to an opponent. While Rokudo Mukuro and doesn't interact with Yamamoto much he respects Yamamoto's strength even if he doesn't initially respect Yamamoto's decision to follow Tsuna. This has more to do with Mukuro's feelings about the mafia and Tsuna's role as a future mafia boss then it does about Tsuna. Dokuro Chrome thinks Yamamoto is kind, which is true of course. Yamamoto is the first guardian to give Chrome a chance instead of mistrusting her for being Mukuro's proxy.