A/N: Thank you's to Jutten for reminding me (some time ago) that chakra is composed of both physical and spiritual energy :)
The two of them stand in the barely lit corridor, the silence between them stretching them uncomfortably. Kagami runs his hands through his hair, messing up the tousled raven locks further as he sighs. "Kara is so much better at explaining this kind of thing..."
Sakumo smiles tightly. "Kagami-sama."
"Okay, okay." The older man drops his hands by his side, his eyes coming up to meet Sakumo's. "You know how chakra works, right? It's made of physical and spiritual energy. Mould it in the right proportions, yada yada, and you get jutsus, right?"
Sakumo nods slowly, unsure where this is recap of chakra theory is going.
"We don't have physical bodies. So where does the physical energy making up our chakra come from?" Sakumo blinks, disturbed that he hasn't considered this before. There is a pained expression on Kagami's face as Sakumo's brain begins to connect the dots.
Kakashi.
"It doesn't make sense..." Sakumo breathes, horrified by the implications. "How can this kind of transfer occur?"
The Uchiha only shakes his head. "I've tested it. I can see the flow of energy with my Sharingan." His brows draw together slightly. "I'll bet your last thought was of Kakashi, as for Rin. If that kind of regret is powerful enough to tie us to this mortal plane, this sort of link isn't impossible."
Sakumo swallows hard, his mouth dry as he frantically sifts through all his memories. "How dangerous is it?" Oh god, if Sakumo had been hurting Kakashi whenever he stubbornly tried to get a jutsu to work... Kakashi often looks tired and listless, but how much of it is because of guilt and grief and the war, and how much of it is because they'd been draining him of his physical energy?
Kagami shifts his weight on his feet and dread pools in Sakumo's stomach. "Well, normally it's no big deal. Kakashi's dealing well enough with two ghosts."
"You can't be sure about that," Sakumo murmurs, the weight of his negligence pressing down on his chest.
Kagami gives him a sympathetic look. "The problem is that when our jutsus sap their energy, it can cause blackouts, fainting spells and the like, which can be fatal in a fight. And if the person is severely injured or exhausted - close to death - losing more physical energy can tip the scales."
The Uchiha's words make Sakumo's stomach turn. He will never forget that moment when Kakashi had collapsed after killing Rin, leaving his unconscious body at the mercy of their pursuers. Kakashi would have certainly died if not for Sakumo's Kuchiyose - and even now Sakumo would have done the same desperate thing - but if that jutsu had killed his son in the process... Sakumo doesn't know how he would have been able to live with himself.
"This doesn't mean you can't use jutsus. You just have to very careful," Kagami says soothingly.
Sakumo stews in miserable silence for a moment before meeting the older ghost's gaze again. "Thank you for warning me, Kagami-sama. I'll make sure Rin knows as well. And... I'm sorry, for earlier."
"Please don't apologise! I was out of line. Hyaku and I fight like cats and dogs all the time and I forget..." Kagami laughs awkwardly. "But there's also another thing I want to say..."
Resisting the urge to bury his face in his hands, Sakumo winces. "Tell me all the bad new in one go, please."
The corner of Kagami's mouth twitches before his humour dims once more. "You and Rin shouldn't follow Kakashi along on his missions."
A/N: double-update so click on the next chapter! :3