Alain de Soisson is a soldier of noble but impoverished birth who serves as a squad leader in the French Guard, the regiment stationed at Versailles. His early life was marked by hardship and a deep resentment toward the high aristocracy, a sentiment born from his family's reduced circumstances and the contempt they faced from the wealthier noble class. He is fiercely protective of his family, a trait that defines his backstory and leads to his demotion; when a superior officer attempted to assault his sister, Diane, Alain struck the man and was subsequently stripped of his rank with no possibility of reinstatement. This act of defiance cements his rebellious nature and his deep-seated distrust of authority.

In personality, Alain is initially presented as a stubborn, impulsive, and passionate individual with a notably misogynistic worldview. He is a man of strong, sometimes brutal, justice and possesses a proud and fiery temperament. This leads him to be openly hostile toward his new commanding officer, Oscar François de Jarjayes, whom he views as a naive aristocratic child. He challenges her to a duel in an attempt to prove her unfitness to lead. However, beneath this rough exterior lies a man of deep feeling, capable of profound loyalty, sensitivity toward his loved ones, and a capacity for self-destructive frustration over his personal failures. His character evolves significantly from a rebellious subordinate to one of Oscar's most faithful and steadfast supporters.

His role in the story is multifaceted. He is initially an antagonist within Oscar's company, leading the other men in their defiance against her. The turning point occurs when Oscar demonstrates her leadership and integrity, earning his permanent respect and, eventually, his lifelong, unrequited love. This unspoken love exists alongside his profound respect for her as his superior officer, creating a complex internal dynamic. His relationships with others are equally significant. He becomes a close friend and drinking companion to André Grandier, offering him support and camaraderie, even as they both harbor feelings for Oscar. The tragic loss of his sister, Diane, who commits suicide after being abandoned by her impoverished fiancé for a wealthier woman, is a pivotal event that deepens his trauma and solidifies his hatred for the inequities of the class system.

Throughout the narrative, Alain demonstrates notable abilities. His swordsmanship is exceptional, considered by Oscar to be on par with her own formidable skill. He is also a capable leader of men and shows proficiency with artillery, leading the cannon fire during the attack on the Bastille. During that assault, he displays immense personal courage, taking a bullet to the shoulder while trying to shield Oscar. His development follows a path from a bitter, rebellious soldier to a dedicated and loyal comrade, shaped by personal tragedy and his growing admiration for Oscar. In the 1979 anime adaptation, after the deaths of Oscar and André, he becomes disillusioned with the Revolution's aftermath and retires to his seaside hometown, living as a farmer who tends the graves of his mother and sister. In other continuations of the story, he continues his military career, only to be executed after a failed assassination attempt on Napoleon Bonaparte.