Xander, who'd been cursing his rotten luck just minutes earlier, was now quite happy with the bargain he'd been able to strike. His plan had been to buy a cheap toy gun and combine it with old soldier fatigues he had at home, but there had been no such guns left, forcing him to alter his plans.
Evidently the shopkeeper had seen him at the bargain bin and after a few minutes conversing and learning he was an old school friend of G-man himself, the Brit had offered him a deal. He had several pieces and accessories in the back that he was unable to sell because they were either faulty or had something spilled on them.
Xander had shrugged at that, he wasn't entering any contests where such details would matter, so he easily agreed.
His haul turned out to be mostly from the Narutoverse, and he'd been able to convince Willow to go as Rin, medic and teammate of his own character, Kakashi. Of course, he wouldn't be the Kakashi from canon, oh no. He'd be making his own alterations, much like the AU fanfictions he liked to read.
His Kakashi character would be a good deal stronger than canon-Kakashi. One very big part of that would be the fact that his implanted sharingan was much better integrated into his body and chakra system. This via a unique seal developed by his teacher, Namikaze Minato and his wife, Uzumaki Kushina. The seal was called the 'doujutsu integration seal', and did as the name suggested, it integrated an implanted doujutsu into the body and chakra system to the point the host could use it as if born with it. They couldn't pass it on, just use it without the usual drawbacks such an implant brought and with only slightly higher chakra demands than those born with the doujutsu.
The second reason for his greater power would be his knowledge and use of seals. As a result of the integration seal, Kakashi understood far better than most the power of seals. His sensei was a master with them and still learning, whereas Kushina was an Uzumaki, who were basically natural seal users. He learned much from them, quickly developing a very useful seal for any shinobi, the 'chakra battery seal'.
Again, as named, it performed. The human body was limited in how much chakra it could naturally contain, even with training there would always be limits. The battery seal was connected to the chakra pathway system, and would effectively double the ninja's chakra supply. Up to five such seals could be placed on someone, granting a ninja up to six times their natural chakra supply.
Because of the integration seal, Kakashi didn't pass out when Rin's voluntary suicide at his hands evolved his still immature sharingan into a mangekyou sharingan, thus learning of Obito's survival and of his new mangekyou sharingan years ahead of canon. The superior integration meant that he could use kamui nearly as well as Obito could, once trained at least.
Eventually, Kakashi managed to awaken tsukuyomi within his mangekyou sharingan and he was working on amaterasu when the Kyuubi attacked. Knowing Kushina was the beast's current container, Kakashi knew something bad had happened, bad beyond 'just' Kyuubi attacking.
That night he sacrificed his sharingan eye using izanagi, to reject the horrific wounds his sensei had suffered while protecting his newborn son from Kyuubi's attack. Kushina, the elder sister he never knew he wanted (much less needed), was beyond even izanagi's ability to reverse, having Kyuubi stripped out of her.
Not suffering fatal wounds, Minato was strong enough to forgo needing to summon the death god to seal Kyuubi in his newborn son, as only an Uzumaki could handle being host of the Kyuubi.
The Uchiha had surprised them by offering a replacement mangekyou sharingan, from one of their fallen. To their collective surprise, the integration seal triggered the awakening of an eternal mangekyou sharingan. Kakashi found that with his new eternal mangekyou sharingan, he could access all three of the mangekyou jutsu, as well as kamui. Because Tsunade herself had been the one to implant the eye, the eternal mangekyou sharingan was even better integrated than the first sharingan had been.
The act of willing sharing one of their precious sharingan meant the Uchiha clan rose significantly in Minato's esteem as well as general standing within the village, enough that there was no attempted coup and therefore no Massacre, leaving both Naruto and Sasuke with family to grow up with.