The Soviet codebreaker who helps Joyce and Murray in season 3 is Alexei.
Dr. Alexei is a Russian scientist involved in the Soviet effort to open a gate to the Upside Down beneath Starcourt Mall. Hopper and Joyce abduct him after discovering the hidden Russian operation in Hawkins.
Murray Bauman translates Alexei’s Russian explanations, allowing the group to understand the purpose of the machine and how it can be shut down. Alexei also explains the Russian facility’s connection to the gate.
While Joyce, Hopper, and Murray continue their mission, Alexei attends the Independence Day fair with Murray. He is killed there by Grigori before the final assault on the Russian base.