Fighting an Animal.

During the first battle arc, the Kokuyo Arc two of Mukuro's "friends", Ken and Chikusa have been going around beating up and removing the teeth of the best fighters in Namimori according to their rank on the list. Gokudera is attacked by Chikusa because he's listed as the third best fighter. Gokudera is able to hold his own but Yamamoto and Tsuna interrupt at the end. Chikusa claims that Yamamoto is Ken's next prey which by process of elimination would make him the second best fighter in Namimori.

"You're Ken's prey, It'll be trouble if I intervene." Ch. 65 Pg. 08

Tsuna and the gang go to Kokuyo is order to prevent anyone else from being hurt in Mukuro's scheme to lure Tsuna out. Yamamoto is among the group who games. While at Kokuyoland, Yamamoto falls into a hole that Ken was waiting in at that's where Yamamoto meets his first real challenge.

Ken is an experiment made by the Estraneo Family in order to try and regain their power in the mafia. The abilities that Ken gained while being tortured under them is the ability to turn into any animal whose "teeth" he has. The system works like old cartridge games where Ken removed the pair of dentures he has on the top of his mouth in order to replace them with a different animal's.

These abilities make Ken a deadly foe who relies on instincts. Ken easily breaks Yamamoto's transforming bat which leaves Yamamoto with only the option to defend his body so that he can play in the upcoming baseball tournament. Yamamoto manages to defeat Ken when Tsuna is pushed into the hole and Ken makes to attack him. Yamamoto sacrifices his arm to Ken's teeth in order to protect Tsuna because he believes now that friends are what's most important in his life (as when Tsuna saved him on the roof). Yamamoto then bashes in Ken's head with the hilt of the transforming bat. The fight shows what Yamamoto's true determination and natural abilities are like.