Sergei is Strahd's brother. Sergei was set to wed Tatyana, but was killed by his brother out of jealousy.
When the waffle crew traveled to Barovia's past, they convinced Tatyana and Sergei that they were in great danger.[1] Tatyana and Sergei got in a carriage and fled the scene, with Strahd giving chase. [2] Strahd captured Sergei and brought him back to Castle Ravenloft.[3]