Altena is a central figure in the shadowy world of the ancient and powerful secret society known as Les Soldats. She holds a high-ranking position within the organization, often referred to as a high priestess, and is the leader of a specific faction dedicated to a mysterious project she calls Le Grand Retour. This project, whose full scope remains unclear to even the other leaders of Soldats, is believed to involve the restoration of the society’s original purpose and the true nature of its legendary assassins, Noir. Her ambition to reshape Soldats according to her vision generates significant opposition from other high-ranking members, who find themselves unable to effectively counter her plans.

Altena presents a deceptively gentle and motherly exterior. She is a tall, slender woman with long light brown hair, usually braided, and sharp purple eyes, often hidden beneath her priestly robes. Her calm and warm demeanor, however, masks a deeply twisted and fanatical core. Her entire philosophy is encapsulated in a single, inverted belief: "If love can kill people, hatred can save them". This principle, born from immense personal trauma, drives every one of her actions. Orphaned as a child in a war zone, she was captured and raped by a soldier, an experience that shaped her worldview into one where large-scale violence, murder, and sacrifice are not only permissible but necessary to save a sinful world. She is not motivated by self-interest or greed; from her perspective, her evil deeds are acts of love and goodwill toward humanity, justifying murder in the name of a higher cause.

Altena serves as the primary antagonist, orchestrating the suffering of the two protagonists from the shadows. It is revealed that she was the one who ordered the murder of Mireille Bouquet’s family and who sent Kirika Yuumura to Japan with her memories erased, setting the entire story in motion as part of her Grand Retour plan. Her ultimate goal is to forge a new, true Noir, an assassin duo she believes will cleanse the world’s evil through murder and revenge, thereby reforming the corrupt Soldats from within.

Her most significant relationship is with her apprentice and surrogate daughter, Chloe. Altena found and raised Chloe within the secluded Manor that serves as her sanctuary, instilling in the girl her own lethal creed. While Altena acts as a loving mother figure to Chloe, sending her on key missions to watch over Kirika and Mireille, her fanaticism ultimately transcends any personal attachment. She demonstrates a chilling willingness to sacrifice anyone—including Chloe, whom she cherished, and even herself—as mere tools on the altar of her grand design. This ruthlessness is further evidenced when she shows little disturbance at the news of Chloe’s death.

Throughout the series, Altena remains a phantom-like presence, her full role and connection to the protagonists unveiled slowly. Her development culminates in the final episode, where she confronts Kirika and Mireille. Abandoning her previous schemes, she demands they kill her to complete the ritual of becoming true Noir. In a final, complex act, as she and Kirika fall into a lava pit, Altena uses her last strength to throw Kirika to safety, accepting her own death with a calm expression as she witnesses the bond between the two assassins. While not a primary combatant, Altena is not defenseless; her weapon of choice is an Mle 1892 revolver, which she uses only in the final episode, demonstrating accuracy and a lack of hesitation when she shoots.