Stéphane Corto, known as Lundi Cortot in the original Japanese version, was a Swiss boy raised by his aunt and uncle in the Swiss Alps during the early 1930s following his parents' deaths. While exploring the Alps around 1932, he discovered a young amnesiac girl, the sole survivor of a plane crash, alongside her pet cockatoo Printemps. He convinced his guardians to take her in, naming her Jeudi and forming an initial sibling-like bond. They grew up together closely in rural Switzerland.

By 1939, their relationship deepened into romance. Stéphane consistently protected Jeudi, especially during confrontations with Count George de Germont. After Jeudi intervened to save Stéphane and Printemps from being shot, the Count attempted to forcibly take her as a concubine. Stéphane orchestrated Jeudi's rescue from the Count's castle, an act that forced them to flee their village and pursue answers about Jeudi's forgotten past.

Stéphane exhibited Chronic Hero Syndrome, frequently risking his safety to help others, such as aiding Clara with medical expenses and assisting Johan in disabling a train bomb. His protective nature extended to resisting Nazi influence as World War II escalated. During their journey to uncover Jeudi's origins, a train bombing separated them. Stéphane later rejoined Jeudi in Austria, where they learned her true identity was Alicia Brendel.

His backstory included a revelation: his brother Jean-Jacques Cortot was an assassin known as "Tarantula". Jean-Jacques initially served Count Germont but ultimately defied an order to kill Swiss General Henri Guisan. Stéphane and Jeudi eventually married in October 1945. Their story concluded with them expecting a child, as revealed in the sequel novel "Emerald Nocturne", cementing their future after wartime trials.