In order to gain the Rain Vongola Ring and the right to be the next generation's Rain Guardian, Yamamoto and Squalo participate in the fourth round of the Ring Battle.
Upon entering the arena, the battle started. Squalo started the battle with an initial strike, flying at Yamamoto. However, Yamamoto dodged with relative ease, stating that his "Daily Image Training" had paid off. With this, Squalo once again took off towards Yamamoto, this time disappearing, indicating that his speed was so great that Yamamoto couldn't catch up with his movements. Yet, in the nick of time, Yamamoto was able to defend himself by using Shigure Soen's Style 7th Defensive Form, Spraying Rain. After that one technique, he was complimented by Dino and Reborn at how far his gone in one week and the fact that Yamamoto was nothing less than a natural born assassin.
However, at that moment, Squalo cornered Yamamoto and just when Squalo was about to deal a damaging blow, Yamamoto used yet another Shigure Soen's Style 2nd Defensive Form: Rolling Rain. After being attacked so much, Yamamoto started to make his move, charging forward and using Shigure Soen's Style 5th Offensive Form: Early Summer Rain. With the attack seemingly effective, Squalo rose up to reveal no damage had been dealt to him at all. Reborn stated the only possibility of this was Squalo reading the move before he used it.
After they exchanged a few words, Squalo charged at Yamamoto and did the same thing as Yamamoto at the same time (supposedly Surging Rain). That way, the first person to see the opponent wins. Squalo however, emerges as the first person to see the opponent and lands a strike on Yamamoto's shoulder. After the strike, Squalo explains why he can read Yamamoto's moves; he had defeated the Shigure Soen's Style a long time ago, much to the surprise of Yamamoto and Tsuna. With this, he then continues to explain that he had heard of the Shigure Soen's Style Technique a long time ago and defeated its successor and two apprentices and they had used the same 8 Forms Yamamoto was using. Squalo ended up cutting them up to ribbons. This was then confirmed by Reborn, stating that his ability to see through techniques is even faster then his Ability to deflect them. With the conclusion of his explanation, the battle once again resumes, with both of them charging at each other.
Squalo got the first strike with Yamamoto easily dodging it. Squalo then attacked again, missing Yamamoto but hitting a column supporting the upper floor. The shards emitted from the strike hit Yamamoto's eye which caused him to be partially blind (one eye only). Ignoring what happened, Squalo continued attacking, forcing Yamamoto to use his 5th Offensive Form: Early Summer Rain. In spite of this, Squalo intercepted and used Attacco di Squalo which sent violent shockwaves to the opponent's body and numbs the opponent's nerves, rendering them immobile. Because of Squalo's incredible strength, the hand Yamamoto received the shock wave with was useless.
Unable to use his Sword, Yamamoto attempted to escape until his hand recovered. However, Squalo found him and was able to stab Yamamoto a multiple amount of times. With Yamamoto near defeat, Squalo mocked Yamamoto saying he would show him all 8 Forms before he died. However, hearing this, Yamamoto's spirit suddenly rose and faced Squalo once again. Because the Shigure Soen's Style always changed for improvements and adaptions, Squalo's 8th Form was different from what Squalo knew, giving him an advantage. In a flashback, it was revealed that the 8th Offensive Form was Pouring Rain, instead of Squalo's Autumn Rain.
Underestimating Yamamoto, Squalo also charged at Yamamoto anticipating Autumn Rain. Yamamoto using Pouring Rain, was then able to deal a damaging blow to Squalo, much to his surprise. It later explains that each generation makes a new technique to pass on so therefore each person may have a different technique. With this, Yamamoto reveals his newly devised Shigure Soen's Style's 9th Offensive Form. Afraid of what would happen, Squalo then rushed at Yamamoto using "Scontro di Squalo", the Technique he used to defeat the ex Sword Emperor. However, Yamamoto's new 9th Offensive Form: Mirroring Rain was able to defeat Squalo. With this, Yamamoto won.
At the same time as the end of the battle, the massive Shark was released because of the level of Water. With no intention of letting Squalo die, Yamamoto asked the Cervello what would happen to Squalo. Their response was As the person who was defeated, Squalo's survival is no longer our responsiblilty. Already anticipating that answer, he attempted to save Squalo but could not because of the massive Shark. Therefore, Yamamoto was officially announced the winner of the Rain Ring Battle.