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1
The first time Obito realized that Kakashi had finally learned to use his mangekyou sharingan was when he returned to his private dimension after a particularly long day, looking forward to the dinner he’d laid out for himself, only to find an arm on his picnic blanket, bleeding all over his sashimi platter. The sleeve that was still attached to it came from an akatsuki uniform. When Deidara showed up to their next meeting missing an arm, "Tobi" naturally volunteered to accompany him back to the river and help him look for it.
According to Deidara, Kakashi had named his mangekyou sharingan technique "kamui." It had never occurred to Obito, after seventeen years, to name his justsu. After all, it was only a nameless justsu for a nameless man from nowhere. But Obito found himself softly muttering "kamui" under his breath when he opened up his portal to retrieve Deidara's arm and place it conveniently in the bushes for the man to find.
2
A few weeks later, Obito was sitting on the his comfortable couch in his kamui dimension, idly flipping through a new manga while waiting for Deidara to finish his fight when he sensed a huge surge of chakra coming his way and immediately teleported out into the woods. When he turned to his private dimension a few minutes later, he found a dust, debris, and ash where his furniture collection had once stood. So much for trying to cozy this place up, Obito thought.
After the grand explosion that destroyed most of his interior design and splashed Deidara’s guts all over his floor, Obito had a few quiet weeks without any disruption, likely because bakashi ran out of chakra, he thought. He used the reprieve to mop up the floor and install some new and inexpensive furniture. He also drew up some scrolls to set up an alarm system to alert him to any new foreign objects appearing in the kamui dimension.
3
This proved to be one of the best decisions Obito had made in a long time. The alert went blaring while he sat waiting for news from Pain’s invasion of Konoha, and Obito was able to save the right unscarred side of his face from the wrath of a nail by blocking it with his favorite cushion.
One day, Obito thought to himself as he pulled the nail out of his now ruined cushion, he would put together a list of damages and send Kakashi the bill.
4
The rest of his cushions were annihilated a few minutes later when a missile appeared from thin air and landed on Obito’s new tatami floor.
But unlike previous times, Obito gasped and fell over in pain as soon as he stepped outside the kamui dimension. His empty left eye socket was hurting. The pain tore through his head as if something had set his nerves on fire. And then, it was over. A sense of cold dread washed over Obito. With the disappearance of the phantom pain, he realized that he could no longer feel the faint ghostly presence of his other eye that he had always been able to detect for the last eighteen years. Obito immediately used kamui again and teleported to Konoha.
Obito stood in front of his other, lifeless eye, invisible to the Konoha ninja around him, watching them cry over the fake dead Kakashi in front of them. Silly Konoha ninja, thought Obito, this isn’t the real Kakashi, none of this is real, this world is not real. The real Kakashi is in the Tsuki no Me, waiting for him. None of this is real.
Instead, he stepped back into his kamui dimension, and began to think about ways to distract Zetsu and kill Nagato. Zetsu will want Nagato alive to revive Madara, but Obito will have to kill him afterwards, the seal on his heart be damned. He is going to rip this man limb from limb.
Obito was still pacing around his dirty, ashen floor, not bothering to clean up what was left of the tatami or the pieces of wood that had been tables and chairs. He was still running through a new plan to speed up the Tsuki no Me so he could see Kakashi sooner rather than later, when he felt the return of a phantom touch on his empty left eye. He went still as he felt the ghost of Kakashi’s fingers stroke over his non-existent eyelid, and all thought of the Tsuki no Me vanished from his mind. Obito let himself, for the moment, simply enjoy the familiar feeling of a warm hand gently cradling his empty left eye, letting Kakashi’s warmth seep into his chakra system, driving away the cold and empty feeling in his stomach. Obito thought he could smell the faint scent of wisteria and honey, drifting into his grey, ashen kamui dimension, with the return of its other owner.
5
Obito was digging through his collection of medical scrolls, preparing for Sasuke’s upcoming eye surgery, when Sasuke’s susanoo arrow appeared out of nowhere and missed his head by a hair.
He sighed, put away his scrolls and sealed them under a barrier, in case Kakashi decided to toss a few explosives in here, and stepped back into the battle.
*
He hadn’t had time to set up new furnishings between Sasuke’s surgery and the war preparations.
At least I swept the floor, he thought, when he felt a punch to his gut from the kamui dimension. This is hardly the kind of thing one should think about in the middle of a battle, he told himself.
And yet, as he pulled Kakashi back into the kamui dimension, a part of Obito’s mind thought mournfully about the nice tatami and the comfortable cushions that Kakashi had set on fire last time. Kakashi could have waited for him there, in comfort, instead of on the cold stone floor, while he dealt with Naruto and the Kyuubi.
*
As Obito bled out on the floor of his kamui dimension, watching Kakashi’s pained expression and shivering hands, it occurred to him that if the plan worked and he successfully absorbed the ten-tails, he would have to clean up the blood here after he finished setting up the Tsuki no Me outside. Maybe Kakashi would lend a hand, he thought. Kakashi always had a talent for water jutsus. Obito had watched him copy and perfect them over the years. Obito smiled at the thought of cleaning with Kakashi. At least Kakashi would now finally understand why he shouldn’t throw trash into our home, he thought as he sent himself back into the outside world.