The Shower That Washes Everything Away

The Vongola family is built around a single boss who has six close advisors known as "guardians". Each guardian represents a different "attribute" of the sky and wield flames of the same color as the attribute they represent. The six attributes of the Vongola guardians are: storm, rain, sun, cloud, thunder, and mist and they are joined by the Vongola boss who represents the sky. As a candidate for being the 10th boss, Tsuna is required to have six guardians prepared for when he becomes the leader of the family. Tsuna's guardians are made up of the young mafioso under his care, Lambo; Tsuna's classmate and the head of the disciplinary committee, Hibari Kyouya; his crush's older brother, Sasagawa Ryohei; his former enemy, Rokudo Mukuro; and his two best friends, Gokudera Hayato and of course, Yamamoto Takeshi.

Yamamoto is chosen as Tsuna's rain guardian and because he is the "type of guy who can't just be left doing nothing". Yamamoto accepts the proposal even though it will take some time away from him playing baseball. When he becomes the rain guardian he officially takes up "the path of the sword" and begins using a real sword as opposed to a transforming baseball bat. He asks his father to teach him and begins to use his father's sword Shigure Kintoki in order to protect his friends.

"The ring of rain... the shower that washes away everything." - Reborn, Ch. 86

Yamamoto's decision to become such an integral part of Tsuna's family changes him. After this arc Yamamoto begins to take things more seriously and consider the "mafia game" that Tsuna and the others play to be just as important as his beloved sport. He even tries to dedicate an equal amount of time to both baseball and swordsmanship.

During the Ten Years Later Arc, Reborn remarks that Yamamoto is not the only rain guardian who had to sacrifice something he loved in order to fully commit himself to the family. The first Vongola Rain Guardian Asari Ugetsu used a similar style to Yamamoto Takeshi and made a similar choice. In Asari's case he was a talented musician whose swordplay was unparalleled. When he learned his friend from overseas, Giotto, The Vongola Primo was in need of assistance he sold the instruments he loved so dearly in order to pay for his passage to Italy and the weapons he would need, one long blade and three short swords, the same type of blades Yamamoto Takeshi uses.

The relationship between the two rain guardians may not be entirely incidental. The dojo owned by Yamamoto Tsuyoshi is known as Asarigumi or the Asari Clan which suggests their could be a connection through the Shigure Souen Style. What is known is that during the final battle against Byakuran the First Generation of the Vongola release the power sealed in the Vongola rings that was locked away when they were broken into halves in order to preserve the strict succession of bosses. After releasing them the rings are restored to their original forms.

After the rings were destroyed, Talbot offers to rebuild them using the remains of the original rings, the condensed forms of their box weapons, and the blood of Vongola Primo; called "Penalty". The rings are rebuilt as rocks that once imbued with the will and flames of their owner take their new form. In Yamamoto Takeshi's case that new form is a necklace that shows his sword and Kojiro and Jirou, the animals that make up his box weapon.