Tsuna is studying with Reborn’s unorthodox methods of teaching when a kid wearing a cow costume outside his window tries to shoot Reborn with his gun but fails, since he already used the ammo. The branch he is standing on suddenly cracks, causing him to fall to the ground. He then rings the doorbell and enters Tsuna’s room. Tsuna is surprised to see him again while Reborn ignores him and easily deflects his attacks. The kid introduces himself as Lambo from the Bovino Famiglia, but cries in the corner when Reborn continues to ignore him. He then throws his grenade at Reborn, but Reborn throws it back at him, which causes him to fall out of the window where the grenade explodes. Tsuna tries to reason with Reborn, but Reborn tells him that he didn’t know him and that he doesn’t associate with the lower ranked Mafia groups' mafioso.
Nana Sawada then calls Tsuna and tells him to be the middle man when Lambo and Reborn fight. Tsuna brings Lambo to the riverside where he found out how Lambo met Reborn; however, Lambo was oblivious to the fact that Reborn was sleeping at the time. Tsuna tries to leave for dinner, but Lambo grabs hold of his leg again, forcing him to take him to dinner. As Nana leaves to deliver something next door, Lambo tries to attack Reborn with his knife, but fails and starts to cry again. He then uses his Ten Year Bazooka on himself. Tsuna is surprised to see Lambo’s future self; however, despite Lambo's older appearance, Reborn continues to ignore Lambo's future self, Lambo using his Electric Horns but failing, and ends up running away, crying. Nana arrives with Lambo, who is back to his present self, who attacks Reborn with multiple grenades, but Reborn deflects them with ease.
Note: Bold + Italicized = First Appearance