We see Silver and Sebek meet for the first time in a flashback where Sebek’s first impression is child-Silver “sleeping standing up with a wooden sword in his hand,” while Silver’s first impression is child-Sebek refusing to accept “such a slovenly human” as his fellow pupil.
In Book 7 Sebek tells Silver, “Don’t you ever make a mockery of my—of OUR teacher again,” which was possibly his first time acknowledging Silver as his fellow pupil.
Silver says that he and Sebek have known each other since they were about 7 and 6 years old and Sebek was the only person back home he knew who was close to his own age.
Silver explains that they have spent a lot of time together since they were young, learning swordsmanship and martial arts from Lilia, and, “You could say we were raised like siblings.”
Silver says that seeing Sebek hard at work inspires him to train that much more.
Sebek has a voice line of, “So Silver purchased a souvenir for Lilia as well. Fine, challenge accepted. We’ll see which of our gifts he likes better,” and it seems they have been raised to be in competition with one another from early on: Sebek sees a flashback from when they would spar together as children where Silver won a point against him, and Sebek demanded a rematch.
Silver responded, “…hm. Okay, Sebek, we can go another round. As long as you don’t mind losing again,” and present-day Sebek admits that he has only ever beaten Silver “a handful of times.”
Silver also beat Sebek to being able to consciously use magic by a year and a half, despite Sebek’s attempts to impress Malleus.
Silver describes Sebek a competitive hard worker who has always pushed him to train. Silver says, “When we first started training together, I found his constant needling annoying…But as time passed, I eventually found that I did not wish to lose to him. That was what I was missing before, when I was only aiming to improve myself and become more like Father.”
Silver says that they did not just compete, but will also join forces to overcome different obstacles: “That sense of accomplishment was better than any I ever felt achieving something on my own. Steel sharpens steel, as they say…”
Lilia says that he once got matching T-shirts as souvenirs that they accidentally wore on the same day, and while it was funny, “they were NOT pleased.”
After badgering Silver into complimenting Malleus in Fleur City (Silver: “I couldn’t get a word in”) Sebek then follows with, “I’m FAR more awestruck than Silver!”
When they both volunteer to stay behind and fend off fire lotuses towards the end of the event Sebek says that he could have handled them all alone, and Silver responds, “I should say the same, Sebek. You could’ve gone with Malleus.”