Amu Chōno is a central character in the second season of the series, known as the GIRS Crisis arc. She is introduced as one half of a popular idol unit called Rummy Labyrinth, performing alongside her partner Luna Yumizuki. Her public persona is that of a professional and somewhat distant idol, but she leads a secret double life under the alias Ace, working for a shadowy organization known as the Company.

Amu has short, blue hair styled to resemble the wings of a butterfly, which is a recurring motif in her appearance. She has red eyes and is often seen wearing butterfly-themed accessories, including a headpiece and choker when performing. Her idol costume is a blue and black frilled dress. In her role as Ace, she dons a short pink wig, a baseball cap, a yellow hoodie, and black jeans to conceal her identity. Her name reflects this motif, as her family name, Chōno, means butterfly field.

The driving force behind Amu's actions is a deep personal tragedy. Her parents were rendered comatose in an accident that occurred during winter, an event which has left her with a lasting emotional scar and a profound hatred for the season. The only memento she has from that time is a single mitten, which she treasures greatly. Her primary motivation for joining the Company and carrying out its bidding is the hope of opening the Stride Gate, a feat that she believes will allow her to bring her parents back from their comatose state.

Amu's personality is complex and evolves significantly over the course of the story. Initially, she is portrayed as highly professional and dedicated to her work as an idol, but this dedication makes her seem distant and unapproachable. She is not used to having friends and has a hard time opening up to others. This coldness is, in part, a mask for the immense压力和 guilt she carries. As Ace, she adopts an arrogant and contemptuous demeanor, specifically targeting Shion Kiba, whose privileged and sheltered life stands in stark contrast to her own struggles. She later admits to finding a perverse enjoyment in forcing Shion to confront the harsh realities of life.

As the story progresses, Amu begins to show more vulnerability. Her relationship with Luna becomes a crucial turning point. Initially keeping Luna at arm's length to protect her from the truth, Amu is visibly moved when Luna retrieves her treasured mitten after it was accidentally thrown away, a gesture that helps break down the walls between them. She is deeply affected when Tokoha Anjou confronts her about her involvement in Ryuzu's plans, highlighting her internal conflict. Ultimately, when she learns that Luna is intended to be sacrificed as a Peacemaker to summon powerful units, Amu rejects this future. She rebels against the Company, choosing her bond with Luna over her goal of saving her parents, which leads to her being fired from the organization.

In the story, Amu serves multiple roles. Publicly, she is a cheerful idol who helps announce and explain the rules of the G Quest to competitors. Secretly, as Ace, she is an antagonist responsible for orchestrating the hostile takeover of Shion Kiba's family corporation. She defeats Shion in a high-stakes Pack Fight, the results of which are broadcast to business leaders, causing him to lose his inheritance and suffer a mental breakdown. Later, she is used by the Company to summon Zodiac Time Beasts, powerful units from the planet Cray, into the real world to cause destruction and further Ryuzu Myoujin's plans.

Her key relationships are central to her development. Her bond with Luna Yumizuki evolves from a purely professional partnership to a deep, personal friendship, ultimately becoming the motivation for her redemption. Her relationship with Shion Kiba begins as one of antagonist and victim, but after she is expelled from the Company, she must seek his help and atone for the suffering she caused him. She also forms a tentative friendship with Tokoha Anjou, who is instrumental in making Amu face the consequences of her actions.

Amu's character arc is a journey from a manipulated antagonist to a remorseful ally seeking redemption. After being cast out by the Company, she is left with nowhere to turn and must contact Team Try3, the very people she helped to harm, to ask for their help in saving Luna from the Company's clutches. Filled with guilt over Luna's resulting comatose state, which was caused by experiments to spare Amu from physical pain, she vows to make amends for her sins. By the end of the arc, she is committed to fighting against the Company and helping to rescue Luna, marking her complete transformation from a villain to a redeemed hero.

As a cardfighter, Amu is quite skillful, a necessity for her role in the Company. She primarily uses a Granblue clan deck, which focuses on the Hollow ability, revolving around pirates and ghost ships. Her ace unit is the Vampire Princess of Night Fog, Nightrose, and later its evolution, Vampire Princess of Starlight, Nightrose. In her role as Ace, her deck is designed for aggressive strategies to overwhelm her opponents quickly. Her abilities as a Peacemaker allow her to use special cards called Depend Cards to summon units from Cray into the real world, though this process places an immense physical strain on her body.