Alain de Soissons is a soldier of noble birth who serves as a squad leader in the French Guard, directly under the command of Oscar François de Jarjayes. Although he comes from an aristocratic family, his lineage is impoverished, causing him to grow up in poverty and be looked down upon by the higher nobility. This background fosters a deep resentment towards the aristocracy and the unequal system of pre-revolutionary France. A defining event in his past occurs when a superior officer attempts to assault his younger sister, Diane. In response, Alain violently strikes the officer, leading to his demotion to the rank of a common soldier with no possibility of regaining his former position due to military regulations.

Alain is characterized by a strong, impulsive, and passionate nature, with a traditionalist and sometimes chauvinistic attitude. He possesses a fierce and raw sense of justice, which often makes him act out against injustice, but this exterior hides a deep sensitivity, especially towards his family. He is a proud man who struggles internally with personal frustrations and failures, which can lead to self-destructive tendencies. A notable habit is scratching his forehead with his left thumb when embarrassed or trying to soften his own insolent words. Despite his gruff demeanor, he cares deeply for his subordinates and is regarded as a reliable elder brother figure within the guard, earning their trust and respect.

At the start of Oscar’s assignment as the new captain of the guard, Alain views her as a naive aristocratic child and is openly hostile, even challenging her to a duel. Although he loses, his swordsmanship is remarkably skillful, considered nearly equal to Oscar’s own ability. Over time, through her dedicated and generous attitude, he opens his heart to her, developing a profound admiration and a lifelong unrequited love for her, both as his superior and as a woman. This creates a complex relationship with André Grandier, who acts as Oscar’s protector. Alain and André are sometimes rivals, sometimes close friends, with Alain eventually becoming a loyal supporter and drinking companion to André. The tragic loss of his sister Diane, who commits suicide after being abandoned by her fiancé, deeply scars Alain and reinforces his hatred for the class system that he believes caused her suffering. His mother also dies from the shock of these events, leaving Alain to honor their memories for the rest of his life.

Alain’s role in the story is that of a loyal subordinate and a representative of the common people’s struggle against the aristocracy. He is a key figure among the French Guard, especially during the revolutionary events. During the attack on the Bastille, he demonstrates his bravery by risking his life to protect Oscar, taking a bullet to the shoulder for her. He also shows notable skill in leading the artillery corps during the assault. After the deaths of André and Oscar, Alain survives. In the original anime adaptation, he becomes disillusioned with the revolution as it turns into an ugly power struggle, leaves the guard, and becomes a farmer living quietly on the coast, tending to the graves of his mother and sister. However, in the manga sequel, Eroica, Alain continues his military career, rises to the rank of general under Napoleon Bonaparte, and serves as his aide. Ultimately, disillusioned by Napoleon’s ascension to emperor, which he sees as a betrayal of the revolution, Alain joins forces with Bernard Chatelet in a failed assassination attempt on Napoleon, for which both are shot and killed.