Leo belongs to First Squad, a Soviet special operations unit of psychically gifted teenagers. He trained with Nadya and others at a military school, honing their supernatural abilities to combat Nazi occult forces during World War II on the Eastern Front. While his specific powers remain undefined, Leo fights alongside the squad against these supernatural threats.

He shares a profound bond with Nadya, marked by deep camaraderie and emotional connection. This culminates in a kiss during a farewell scene, revealing their personal attachment amidst the war.

Leo perishes early in the conflict during an air raid that shatters First Squad, leaving Nadya as the only survivor. His physical presence in the main narrative ends with this death.

Afterward, Leo exists within the afterlife realm, the Gloomy Valley. Nadya encounters him there while recruiting deceased squad members for a crucial battle against the resurrected Teutonic Knights. Answering her call, Leo agrees to return temporarily to fight, demonstrating his unwavering loyalty to the Soviet cause and his former comrades. This aid marks his final appearance.