The battle was rough.
Some shinobis came from nowhere and embushed them, trapping them to being surrounded and outnumbered. Usually, it would not be a big deal for the Zombie Combo, but this day has already been a big shitty day, with earlier difficult fights where Kakuzu had already lost two of his hearts. And now he's down to one, his mask partially destroyed while he's really, really pissed off. It was all Hidan's fault, not being able to shut up for once and not announce their presence to any ninja in a 20 kilometers perimeter.
A particularly annoying but so familiar sound made him turn his head to see the religious psycho laughing over a shinobi trapped in his threat. He then, impaled himself in the heart and started shouting some incomprehensible things about his pathetic god, Jashin.
Kakuzu sighed. Of course Hidan would be talking about Jashin to a dead person.
So concentrated on checking if his stupid partner won't be beheaded, he senses the enemy behind him a little too late. He already knew that he wouldn't be able to avoid this attack when he faced his opponent. Immediately, Kakuzu thought about Hidan, how this moron will be happy to see him finally die. But he also worries about the fact that Hidan would be lost without an authority figure, without any indications. Who would sew again his limbs when he's gonna loose them ? Because he will lose them, that's an evidence. Seeing the sword inching itself closer to him, he then thought about the fact that nobody would miss him, a rogue ninja, who killed so many people for money, for his own benefit. Not even his own partner.
Kakuzu closed his eyes, prepared to receive the impact.
Which never came.
Confused, the old man opened his eyes and the scene he witnessed made him more confused about what he believed before. There was Hidan, his partner, impaled on the enemy's sword, swearing and aggressively threatening his opponent, who was already dead. The albinos then turned his head towards Kakuzu and angrily shout at him to just move his ugly ass or he will die. The bounty hunter just looked at his loud partner and without letting him finish his complaints about him being stupid, said one thing. One little thing which later, would change everything between their duo.
"Thank you."
He then turned over the opposite side to not look at Hidan's surprised face and visible confusion in his eyes. But he didn't miss the mumbling about him being a dick, miserable old man who could not be careful. Without a second though, Kakuzu got back into the battle, without looking back to the silver head still impaled. He just couldn't let his partner know why he turned back.
Because under his mask, Kakuzu was smiling.