Sudden contact with Isamu would be startling, had Kazuo not known him so well. The hand falling on his long hair was too familiar to provoke anything but a smile. "Sensei" - he nodded quietly as he watched and listened: despite Isamu's words, his eyes were fixed on his hand, and grew a bit larger as it took up the eel and transferred it to his fingers, which drew to his beforehand, seemingly by themselves. Only reason he was doing nothing was because of Isamu's power and status: Nothing he could imagine could as little as hurt Isamu.
Our imagination will grow wilder someday...
The eel on Kazuo's hand swirled all over it, leaving cold and pleasant sensation all over it. Kazuo focused on its head and it returned eye contact, seemingly acknowledging it. This small thing could be his war companion. Such might in such a small thing... - Insofar as at no less than what would do for Shinobi's companion.
Kazuo almost didn't see the scroll before him, transfixed upon the eel. Slight smile from his part made the eel return favor: by swirling all over his hand, then arm and then back to his finger. The sensation was tingling, and the genin's body shook slightly. Now, Kazuo felt subtle but seemingly profound lightning chakra, harmoniously seeping into his arm. His senses hightened, his body invigorated, his mind sensing the joy of the eel. It took to his liking. Only then did he noticed the scroll. The names of his two sensei's were almost watching him through their bloodied letters. He understood enough of the ways of such things: A contract written in blood. It would let him summon these eels at will, if blood is drawn from.... right, the hand that wrote the contract. Doing so by writing on a scroll was common, tattooing a contract less so. Once signed, it persisted beyond death.
The permamence of such an action made Kazuo hesistant, but this was the one time the voices were silent. Nothing was left but to shake the eel to Kazuo's left hand. As soon as his right reached his left, the eel leaped to his left arm, apparently understanding of what was to happen. His left hand was fully free to draw the Kunai to puncture his right index finger, travelling to the spot beneath Sumato's name to seal the deal.
Blood sealed the deal, as it formed Kazuo's name beneath Sumato's. Once that was done, the feeling of the eel swriling around his arm expanded, as it seemed to celebrate the occasion. Such an eel could be summoned by him now, with a little of his blood. Or a lot more, for an adult eel, or two. Or more, if he was truly generous. Summoning demanded offering in blood: its inherent power could not be easily aped. But what if Kazuo wore his blood in blood bags, thereby extending his ability to expend it? Already he wondered how to game the system, optimize.
"Is it only blood they'll accept?" - he asked Isamu after a while.