This character is only featured in the Multiplayer Mode of Grand Theft Auto IV. Therefore, this character may not be canon to the main storyline.
Sergey is a character in the GTA IV era. Sergey is a member of the Petrovic Family. He is young and possibly inexperienced, which may be the reason that he got arrested. When the LCPD is moving Sergey to jail, Konstantin Petrovic orders Sergey to be killed for getting arrested.