We get many examples throughout the game of Sebek’s apparent self-confidence: he says that the prefect should be astounded by the rapidity of his personal development and explains that he has overcome his aversion to bitter vegetables because he can overcome any hardship.
When Ortho tells Sebek “I’m glad you’re here!” during Fairy Gala IF Sebek responds, “You should be.”
He also declares that he will be taking first place in the Harveston race, crush anyone who challenges him during New Years and after explaining why he can understand a dialect of fae he follows with, “ARE YOU NOT IMPRESSED?”
Deuce tells him that “Having confidence is fine and all, but you shouldn’t make things harder for others. That’s just being obnoxious,” and other characters also comment on how condescending Sebek can sound such as Grim, Ace (multiple times), Jack, Leona, Vil, Azul, Idia and Trey.
Examples we have seen include Sebek declaring his superiority over others during Fairy Gala IF, Harveston, and New Years, while during Book 7 he calls Grim and the prefect “the bottom of the Class A barrel.”
It is possible that Sebek’s supposed self-confidence is hiding insecurity: Lilia says he suspects Sebek was “terribly anxious” until receiving his acceptance letter to NRC, as his magic manifested late and he might have question if he was a worthy enough mage to attend the school.
Lilia explains, “He’s a calm, clever boy… when Malleus isn’t involved, that is. So l’m certain he could objectively evaluate his own magic.”
Silver says that Sebek had been devastated when Silver’s magic manifested first, and Lilia says that Sebek compensated for his delayed magic with smarts and stamina, working doggedly to improve the skills he had: “I suspect that training is what really got him here. You won’t find many people as hardworking as he. Sebek’s conviction is a powerful thing indeed.”
Twisted Wonderland Reference Note 