Time Travel - Not a Hatake
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Sakumo couldn't say that he had known Hake Kama for a long time in comparison to Jiraiya, whom he's known since he was four, but he felt as if he could claim the man as a friend- a good one at that.
The twenty-three year old had shown up during the climax of the Second Shinobi War, seeking refuge for himself and the displaced civilians he had decided to escort (as a shinobi who's village had been destroyed during the fighting) to the safety of Konohagakure. To the dismay of the civilians the then seventeen year old had been protecting, Kama had been dragged in for questioning; Sakumo didn't know exactly what happened only that it involved the Hokage, Yamanaka Clan Head and Uchiha Clan Head (as the teen possessed a Sharingan)- the end result though was the three stating Kama could be trusted and that everything had been explained to their satisfaction.
To appease the Council, the teen had allowed Jiraiya to place a seal on him that would kill him should he betray Konoha.
It wasn't until his wife had been three months pregnant that Sakumo had met Kama, the Nidaime assigning him as the teen's partner in a way to a Jōnin-sensei as another appeasement to the council when Kama decided to continue his life as a shinobi. It was something he had not known how to feel about as the last of his genin cell had died recently and he viewed Kama as trying to take their place.
Those first few months were hectic as they both tried to feel the other out, learning about one another as they attempted to mesh together as teammate.
At first, the entire thing was exceedingly frustrating as Kama had been withdrawn and refused to so much as look at Sakumo let alone speak to him- something that, as he knew the teen interacted cheerfully with others, made him contemplate going to the Nidaime for a replacement so to be placed somewhere he was of use with a war going on. Then, as if he knew of these thoughts, Kama seemed to gather himself and look him straight in the eye for the first time and cheerfully tell Sakumo that his 'chakra control sucks, you tend to bleed when you use jutsu in a way that acts as a warning'.
From there it was a matter of getting their fighting styles to flow together; Sakumo's being more flamboyant and intimidation-based while Kama's was more subtle, analytical and based on being underestimated. They settled on a simple yet deadly tactic of Sakumo being the distraction whilst Kama went for the kill- something that earned the teen the nickname the 'White Fang's Blood-Soaked Shadow'.
And Yes, Kakashi is irritated that he doesn't get his own nickname.