The Only One Who Knows.

During the Ten Years Later Arc, Yamamoto is told a secret that none of the others, not even Tsuna know. Reborn tells him the truth about the Acrobalenos and their curse. Personally, many fans would probably wonder why Reborn told Yamamoto as opposed to Tsuna or any of the other guardians.

Reborn told Yamamoto after training him in preparation for his fight against the Millefiore. It was Yamamoto's reward for being able to hit him once, cutting his hat. Why Yamamoto though? Reborn trained Tsuna for so long, and Lal trained Tsuna as well, Colonello trained Ryohei but none of them told the secret. I believe that Yamamoto Takeshi was told because of his past and personality. Yamamoto, still claims that he believes it's a Mafia Roleplaying Game however due to his actions otherwise, such as saving Squalo at the end of their battle, seem to say he knows this is more than a game, it's a life or death situation.

I believe that Yamamoto's claim that he doesn't realize it's not a game is part of why Reborn told him. Any of the other guardians would most likely show emotion, fear, anger, sadness, any of that over such a grave secret. Because Yamamoto won't break his secret that he knows it's real he can respond to the secret as a "good story" or "imaginative" and continue on with his life, smiling as he goes. For Yamamoto to know and believe it's part of the game also makes it guaranteed that Yamamoto won't tell, unless Reborn says he can, because it would compromise the game. This makes Yamamoto the perfect person to tell.

But why share the secret in the first place. I believe that the secret of the Acrobalenos is going to be critical when they fight the Millefiore later and for Yamamoto to tell the others, as a comforting and carefree figure, rather than Reborn whose serious nature may aggravate the situation in which the others were told.