Sergei Beznikov is a Heartrender who resides at the Little Palace.
Shadow and Bone
Sergei is introduced to Alina Starkov after her arrival at the palace, at the occasion of her meeting with the King. He and Marie argue on who should walk Alina into the room, though the Darkling interrupts their dispute by stating he will be the one doing it.
Later on, Sergei is part of the little group of Grisha Alina sometimes spends time with, the closest thing she has to friends at the palace.
Siege and Storm
Sergei is now in a relationship with Marie. He has been assuming a few of the Darkling's responsabilities since his attempted coup and subsequent disappearance. He is one of the few Heartrenders not to have left Os Alta and is locked-up with the other remaining members of the Second Army within the Little Palace.
When Alina comes back to the palace to take the lead of the Second Army, Sergei opposes her, claiming that as a Corporalki he is more qualified than her to lead the army. The argument almost takes a dramatic turn when an angry Alina unleashed the Cut towards Sergei, though she realizes what she's doing at the last minute, resulting in the Cut hitting the domed roof instead, breaking it in half.
The tension between Alina's supporters and her opponents eventually disappear, as teamwork proves to be their only chance to defeat the Darkling.
Sergei is one of the few to survive the Darkling's attack on Os Alta. Obviously distraught at the death of Marie, Sergei has to be carried away from her dead body to the safety of the domed hall.
According to Mal, Sergei is still in shock days after the battle. He and the other survivors follow Alina to the White Cathedral.
Ruin and Rising
Sergei lives in the White Cathedral, his mental and emotional state getting worse and worse after Marie's death. Together with the other grisha and Alina and Malyen he escapes and meets Nikolai, who leads them to his headquarters, the spinning wheel. Due to his miserable mental state, Alina offers him to go to Duva in order to rest and recover. He happily agrees and leaves the spinning wheel on board of the Ibis, one of Nikolai's flying ships.
After leaving the spinning wheel, he betrays Alina and Nikolai, telling one of the Darkling's Oprichnik where Nikolai and Alina stay. As a result the Darkling attacks the spinning wheel and punishes Sergei in front of Alina's eyes for his betrayal, having his shadow warriors cutting off Sergei's legs, arms and his head.