Kei Kuruma is the sole female protagonist among the four central characters engineered with the ARMS. Her physical appearance is that of a teenage girl with brown eyes and black hair that reaches her chest. Unlike the other three individuals who received their ARMS, Kei has been aware of her nature her entire life. She was raised from childhood by the Bluemen, a secret organization that created the ARMS to combat the Egrigori, and was trained as an elite soldier. Her upbringing was harsh and isolating; because many within the Bluemen regarded her as a weapon rather than a person, she grew up feeling extremely lonely, bitter, and vicious.

Her personality undergoes significant transformation after she meets Ryo Takatsuki, Hayato Shingu, and Takeshi Tomoe. These fellow ARMS subjects become the only people who truly understand her, and through them, she learns the meaning of friendship and teamwork. Kei often serves as the voice of reason within the group, tempering the aggressive impulses of her comrades and helping to unite them toward a common objective. Her relationship with Hayato deepens over the course of their shared missions, eventually developing into a romantic involvement.

Kei's role in the story is directly tied to her mission from the Bluemen. After the destruction of the Abumisawa village where the ARMS children were created, she is dispatched to recruit the other three subjects, using the revelation that Ryo Takatsuki's girlfriend Katsumi Akagi is still alive as leverage. Despite the Bluemen's expectations, the group refuses to serve the organization and instead chooses to follow their own path. Throughout the narrative, Kei grapples with her identity as both a living weapon and a human being, eventually reconciling these two facets of herself as she forms genuine bonds with her teammates.

Her ARMS is named the Queen of Hearts, and unlike the combat-oriented powers of her companions, it is concentrated in her eyes, which were replaced with cybernetic ARMS during childhood. The Queen of Hearts functions not as an offensive weapon but as an impartial arbiter, possibly representing Alice's objectivity as a scientist rather than aligning with either the White or Black Alice factions. This ARMS grants Kei an array of powerful sensory abilities, including magnification, infrared vision, and predictive combat analytics that allow her to anticipate a punch before it is thrown or track a bullet's trajectory before it leaves the barrel, enabling her to avoid attacks. She can also activate, enhance, or deactivate the ARMS of her teammates, though mastering the volatile Jabberwock proves challenging. The Queen of Hearts' nanomachines accelerate her healing even when not in a transformed state, and she has affectionately nickmed these microscopic entities her nano-babies.

The Queen of Hearts possesses several ultimate abilities that manifest as Kei's understanding of her powers deepens. Its final form is a magnificent woman composed of light, unlocked not by a wish for power but by choosing Light when asked Do you want Light?. This form demonstrated the capacity to stop and reverse the out-of-control transformations of the Jabberwock, White Knight, and White Rabbit. Another evolved ability is the Mirror of Aegis, a powerful shield that can block and reverse any attack with equal or greater force. Most significantly, the Queen of Hearts possesses the ultimate failsafe: the ability to permanently deactivate all ARMS units descended from Alice, including Alice herself, by simply uttering the word Destroy. However, this action would also result in the death of all ARMS hosts, including her friends. Whether Kei possesses the mental strength to ever use this ability remains unknown. During the final battle against the Bandersnatch, the Queen of Hearts' faceplate lifts to reveal the visage of White Alice, a testament to the shared origins of Kei and her ARMS.