Arnval, who is almost always called by her nickname Ann, is the primary protagonist of the Busou Shinki anime series. She is an Arnval Mk. 2 model, a type of small, intelligent robot known as a Shinki, designed to serve and assist a human master. Ann stands out as the first Shinki belonging to her master, a high school boy named Rihito, and she is often depicted as his most cherished and favored partner among his collection.

In terms of appearance, Ann is a blonde-haired, blue-eyed android who typically wears a white and blue set of armor that reflects her angelic motif. She is also almost always seen wearing a pair of headphones, which is a consistent part of her character design. Her personality is defined by her kind, loyal, and determined nature. As her angelic theme suggests, she is generally gentle and devoted to those she cares about, especially her master. However, this model is known to have a specific programming quirk. When Ann feels intense jealousy or possessiveness, particularly regarding her master's affections, she can temporarily become a violent and possessive berserker, a stark contrast to her usual calm demeanor. This internal conflict between her caring nature and her volatile potential is a core aspect of her character.

Ann's primary motivation is the happiness and well-being of her master, Rihito. She is deeply devoted to him and takes her duties as his partner very seriously, whether that involves helping him with daily tasks, protecting him from danger, or striving to be the most important Shinki in his life. Her role in the story is that of the central figure among Rihito's Shinki, which also includes the Altines model Ines, the Altlene model Lene, and later the battle-oriented Strarf model named Hina. Ann often acts as a stabilizing and guiding presence within this small group, trying to maintain harmony and encourage teamwork, even as she privately competes for her master's attention. Her key relationships are with these fellow Shinki, with whom she shares a home and a sense of camaraderie, and with Rihito, whose bond with her is the emotional core of the narrative.

Throughout the series, Ann shows development in her ability to balance her possessive instincts with her responsibilities as a partner and friend. Early episodes highlight her jealousy and the berserker rage triggered by threats to her bond with her master. Over time, she learns to trust her fellow Shinki and work alongside them, understanding that her master's care for others does not diminish his care for her. Notably, in one episode, she experiences a dream in which she wakes up as a full-sized human, and in another, her Shinki body is infected with a dangerous virus, both scenarios testing her identity and her connection to Rihito.

Regarding her abilities, Ann is a capable combat unit designed for battle. As a Busou Shinki, she can equip various weapons and armor to fight. She is shown to be a formidable fighter, able to hold her own against other Shinki models. Her anger and emotional state can directly enhance her combat power, as seen when she overwhelms an opponent after a treasured memento is damaged. In one instance, her flight pack, equipped with attack drones, is used by another character to secure a victory in a tournament, highlighting the tactical value of her equipment. While her core programming as an Angel Type emphasizes support and loyalty, her combat skills and the berserker mode triggered by jealousy make her a uniquely unpredictable and powerful ally.