Sergei Kravchenko is a Belarusian mathematician. He was co-winner of the Litvinchuk Medallion of Honour along with his older brother Maxim for his work on the twin prime conjecture.
Sergei Kravchenko worked as a security consultant for Isaac Vavasor and attached the tracking device to George Burgess's briefcase containing the Vavasor Papers that enabled him to track Tom Winscombe down to the Dot Chan offices. However, once Ali MacLehose had located and removed the device, Tom managed to hide it in a lorry load of bananas, thus keeping Sergei off their backs.
When Tom subsequently had to arrange an exchange of the briefcase to release Dorothy Chan from the Gretzky gang, Arkady Morozov arranged for a friend of his to attach a tracking device to it. The friend turned out to be Sergei, who instead inserted high explosive into the case in order to gain revenge for the murder of his brother Maxim.
Following all this, the whereabouts of the Vavasor papers are unknown.
After the episode with the Gretzky gang, Sergei is forced to lie low for a while and is unable to return the papers to Isaac. However, his flat is broken into and the papers are stolen by members of the Petrov mafia family. He decides to offer himself to the Petrovs as a consultant in order to get the papers back. Unfortunately, he is rumbled by the Petrovs and killed. The papers now end up in the hands of Rory Milford instead.