Saruhiko Fushimi and Yata Misaki met during their first year of middle school. They met when Misaki stumbled on Saruhiko being beaten up by some upperclassmen. While Yata tried to save him they both ended up getting beaten up. That should have been the end but Misaki was relentless and eventually the casual disdain that Saruhiko had everyone around him wasn't there when it came to Misaki. Though their personalities and their home lives were completely different their personalities were similar. Neither was close with their family which made the choice to move in together after middle school an easy one. They left home and cut off from their families lived in poverty in a one room apartment in a multi-use building.
"The part where he hates to lose is unexpectedly similar to Yata." - Tatara Totsuka, Days of Blue Ch. 5 Pg. 22
Saruhiko's friendship with Yata Misaki was one of the first positive relationships that Saruhiko had. They were able to be successful as friends despite Saruhiko's limited ability for human interaction because he was able to understand Misaki. Yata Misaki wears his heart on his sleeve. He thought the world of Saruhiko and because of Yata's confidence in him, Saruhiko was able to believe in himself. Saruhiko puts up a cocky and apathetic front but his relationship with Misaki shows just how deeply Saruhiko's sense of self worth is connected to those around him.
"You know Saruhiko, I thought of you as one of my important comrades." - Yata Misaki, Memories of Red Ch. 11 Pg. 30
Misaki and Saruhiko begin to fall apart when they almost die and are saved by Homra. Saruhiko realizes that Misaki's belief in him isn't enough to change things. They join Homra in order to get stronger for that purpose. They become the only members of Homra to share their marks in the same location, just below their clavicle on the left side. After joining Homra however, something changes and suddenly Saruhiko isn't the one Misaki looks up to. This shakes Saruhiko because he no longer understands his place in Misaki's life. He doesn't see a place for himself in Homra either. Suddenly Misaki, the one person who shared Saruhiko's wants and goals didn't need him anymore. Misaki didn't understand Saruhiko and though it's not clear if Saruhiko realized it he never really did. That makes it easier for Saruhiko to leave Homra.
"Because it's stupid. Did I smash this thing you call pride, Misaki?" - Saruhiko Fushimi, Memories of Red Ch. 10, Pg. 15
Misaki is the only one to call Saruhiko a traitor to Homra and that's because Saruhiko left Homra but he betrayed Misaki or at least that's how it felt. Saruhiko joined Scepter 4 because he was unhappy in Homra and Munakata Reisi gave him a better offer. It was his actions towards Misaki, burning his mark from Homra and mocking how Yata felt about Homra that made it a betrayal. Saruhiko only does this because it's his only way of regaining control from Misaki. He feels abandoned and by burning him he becomes the one who abandons him. After that every time they meet Saruhiko becomes manic and obsessive, mocking Misaki in order to get him fired up. Why? Because if Saruhiko can't be Misaki's best friend he'll be his worst enemy because at least then Misaki will contiue to look at him.