She is the only girl of the four protagonists. Unlike the others, Kei has known about her ARMS 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 saw her as a weapon and not a person, Kei grew up extremely lonely, bitter, and even a little vicious. This all changes when Kei meets Ryo and the other ARMS subjects, the only people who truly understand Kei. 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 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". The Queen Of Hearts' final form is a magnificent woman made of light. Kei is still discovering all the abilities of her ARMS. When the Queen of Hearts displayed its ultimate form for the first time in volume 10, it was able to stop and reverse the out of control transformations of the Jabberwock, Knight, and White Rabbit. In Volume 16, the Queen of Hearts evolved to gain the ability "Mirror of Aegis", which appears to be able to reflect an attack with equal or greater power than was used. In volume 18 the Queen of Hearts reveals to Kei that she holds the ultimate endgame weapon of all the ARMS units. She merely has to say the word "Destroy" and all ARMS units that descended from Alice, and even Alice herself, will be permanently deactivated which will also result in the death of all of the hosts including her friends. Whether Kei has the mental strength to actually use this ability is unknown.

(Source: Wikipedia)