Kuruma, Kei

Kei Kuruma is the sole female among the four protagonists. Unlike the others, Kei has been aware of her ARMS throughout her entire life. She was raised by the Bluemen, the organization that combats Egrigori and originally implanted ARMS in the four teens. Kei was raised and trained as an elite soldier, but because many of the Bluemen viewed her as a weapon rather than a person, she grew up extremely lonely, bitter, and even slightly vicious. This changes when Kei meets Ryo and the other ARMS subjects, the only people who truly understand her. With them, she finally learns true friendship and teamwork. Kei and Hayato seem to be growing closer as time goes on. Whether this will develop into anything romantic remains to be seen.
Kei's ARMS is called the Queen Of Hearts and is concentrated in her eyes. Kei's ARMS is an array of powerful sensors, including magnification and infrared. Kei can figure out a punch before it is thrown or a bullet's course before it leaves the barrel and avoid them. Kei can also activate, enhance, and deactivate the ARMS of her teammates. The Queen of Hearts' nano-machines allow Kei to heal from injuries much faster than a normal human even when not transformed. Kei has nicknamed her nano-machines her "nano-babies."

Spoiler