Sergei is almost always shown to be calm and composed.
Around ten years ago, Faust, Sado, Sergei, and Fogg attacked the Oni Village. Sergei killed Dudley Miload and confronted Roswaal pretending to be Dudley's follower, though he was defeated by the former and had all four of his limbs broken.
Mana Poisoning: Sergei is able to contaminate the atmospheric mana in the nearby vicinity and make it poisonous. As Oni absorb mana through their horns to strengthen themselves, he was their worst possible opponent. This ability is also effective against mages as it makes it harder for them to use the mana.
Explosion Technique (爆裂術式): Seigei is able to use the Explosion Technique, however he admits that he's terrible at it.