Lune Vanquish hails from a remote border region of the Kingdom of Soleil and serves among the kingdom's military mecha pilots. He operates as part of a brother-sister team with his twin, Yuni Vanquish. Lune possesses a calm and measured personality, providing a stabilizing counterweight to Yuni's brighter, more impulsive nature. He consistently supports his sister, reining in her rash tendencies and offering reliable backup, defining their dynamic as her steadfast supporter.

Physically, Lune has black hair and golden eyes. Recognized as an excellent pilot, his true strength lies in collaboration and support rather than solo command. He excels in team-based missions and coordinating with other units, contrasting with his sister's greater initiative.

Early in the series, Lune and Yuni express skepticism regarding Princess Yuki Soleil's capability to rule. This leads to a confrontation with her childhood friend, Joshua Ingram, culminating in a simulated Capture the Flag mecha battle won by Joshua with the princess's aid. Following Joshua's death in a border conflict, Lune adopts Joshua's characteristic catchphrase, "Passion and Will!", as an act of respect and symbolic tribute.

As the war with the Grandiga Empire escalates, Lune and Yuni volunteer for a high-risk, potentially suicidal mission to aid Princess Yuki. This decision comes during a critical moment after political officer Izana Langford transmits vital intelligence instead of escaping capture and death, highlighting their loyalty despite the personal cost.

Lune's journey reflects a shift from initial doubts about the monarchy to active defense of the kingdom, marked by adopting Joshua's motto and embracing extreme sacrifice alongside his sister.