Okay, first off, just because I have to get it out of the way: hot like fire.
Yamamoto is another of those contradictory characters that’s so much fun. On the one hand he’s laid back and cheery. He smiles at everyone, including Hibari and his opponents. On the other, he has, starting in post60, a definite ferocious streak. The way it’s most often characterized is as a will not to lose, and when he’s in that mode he’s downright alarming–still smiling sometimes but in a very different way indeed.
Yamamoto’s fierce streak is paired with a protective streak, and sometimes protectiveness will bring out the fierceness. Consider Yamamoto’s fight with Ken, in the Mukuro arc. He looks fierce after his bat/sword gets broken, and engages readily with a strong and uncanny opponent, but he doesn’t fight all out. Not until Tsuna is in danger. That’s when he throws it all down, even to the point of sacrificing his arm to get his opponent in range. And in more general terms, a good half of Yamamoto’s entrances seem to involve rescuing someone, most often Tsuna but sometimes other family members as well.
All of this, however, is also matched up with a set reluctance to kill. Squalo, during their fight, asks several times if Yamamoto is screwing around or not taking him seriously enough because he keeps striking with the spine of his sword instead of the edge. It seems like quite a legitimate question, given that Iemitsu’s evaluation of Yamamoto, at the start of this arc, is that his fighting spirit lacks the harshness necessary to fight the likes of the Varia. I find it interesting that Yamamoto never actually answers Squalo; he simply keeps fighting his own way.
So Yamamoto’s intensity is consistently tempered. Going all-out, for him, I speculate, has little to do with bloodlust. Besides his protectiveness, I think what drives, and equally gentles, his ferocity may actually be the same things that make him so suited to athletics: a team spirit and a will to win on the gameplay level. Let us consider. On the one hand, I get increasingly suspicious of his apparent cluelessness about mafia doings as time goes on. If he’s serious, then either he’s stunningly, moronically oblivious to the stakes involved in these encounters, which sorts oddly with his general perceptiveness and ability to do things like redirect uncomfortable conversations, or else he’s psychotic enough to consider grievous bodily harm reasonable stakes for a game. I’m more inclined to think he has an arcane sense of humor and is enjoying the looks on people’s faces when he makes like he doesn’t quite get it. On the other, I think his tendency, even post60, to interpret things as a game points to something significant: he’s that intense even over games. The flipside of this is, of course, that his understanding of fighting and winning is shaped by and bound up in forms that are not life and death.
I imagine this is why he drives people like Squalo, and possibly Hibari, to absolute fits. He doesn’t have the same scale of measurement they do for winning and losing.
I think this is also why his patience with Gokudera finally snaps. Yamamoto clearly understands team play, and the more dire the situation the more strongly he seems to return to that touchstone. Strongly enough, indeed, that he cannot just stand aside and let Gokudera’s stubbornness be its own punishment; instead he’s driven back to insist on the point.
This is not exactly to say he’s the sane one, because anyone who jumps that readily into the path of weaponry even before he’s trained and looks for training just to even up the score with explosive, sword-wielding maniacs doesn’t really qualify as sane. I do think, though, that he’s probably the most stable of Tsuna’s Guardians.