Shadow and Bone
Sergei was introduced to Alina Starkov after her arrival at the palace, at the occasion of her meeting with the King. He and Marie argued on who ought to walk Alina into the room, though the Darkling interrupted their dispute by stating he would be the one doing it.
Later on, Sergei was part of the little group of Grisha Alina sometimes spent time with, the closest thing she had to friends at the palace.
Siege and Storm
Sergei was in a relationship with Marie. He had been assuming a few of the Darkling's responsabilities since his attempted coup and subsequent disappearance. He was one of the few Heartrenders not to have left Os Alta and was locked-up with the other remaining members of the Second Army within the Little Palace.
When Alina came back to the palace to take the lead of the Second Army, Sergei opposed her, claiming that as a Corporalki he was more qualified than her to lead the army. The argument almost took a dramatic turn when an angry Alina unleashed the Cut towards Sergei, though she realized what she was 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 disappeared, as teamwork proved to be their only chance to defeat the Darkling.
Sergei was one of the few to survive the Darkling's attack on Os Alta. Obviously distraught at the death of Marie, Sergei had to be carried away from her dead body to the safety of the domed hall.
According to Mal, Sergei was still in shock days after the battle. He and the other survivors followed Alina to the White Cathedral.
Ruin and Rising
Sergei lived 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 escaped and met Nikolai, who led them to his headquarters, the spinning wheel. Due to his miserable mental state, Alina offered him to go to Duva in order to rest and recover. He happily agreed and left the spinning wheel on board of the Ibis, one of Nikolai's flying ships.
After leaving the spinning wheel, he betrayed Alina and Nikolai, telling one of the Darkling's Oprichnik where Nikolai and Alina stayed. As a result the Darkling attacked the spinning wheel and punished Sergei in front of Alina's eyes for his betrayal, having his shadow warriors cutting off Sergei's legs, arms and his head.