Isaak Andreyev was a First Army soldier when he met Nikolai Lanstov. The prince encouraged Isaak to use his talents for languages, although Isaak was hesitant he took the job to supply money for his family. He took up random labor jobs in his home town while healing a wound he had acquired in battle when he received a letter from Nikolai offering Isaak a job as a member of Ravka's royal guard. With this work Isaak was able to better care for his mother and sisters back home and become fluent in Shu, Zemeni, Kerch, and Suli languages.
When Nikolai and Zoya go missing in the Fold, Isaak is asked by the Triumvirate to temporarily present himself as Nikolai. Isaak's loyalty to Ravka and his fondness of King Nikolai cause him to accept, although he requires some time to act and sound as Nikolai. Genya tailors Isaak's physical features to resemble the King (as Isaak was chosen due to his similar height).
Isaak agrees to their request and acts as Nikolai for a ball that the crown of Ravka is throwing to find Nikolai a bride. At the ball he falls in love with a female assassin that is disguised is to look like a Shu princess. She mistakes him for Nikolai Lanstov due to him being altered to look like the Ravkan prince and attempts to kill him (with a Fjerdan dagger) and then attempts suicide. She succeeds in killing Isaak but is saved by the Yul-Bataar twins. She is then held for interrogation not specified in King of Scars.