Ryo's neighbor and classmate, and is very kind and compassionate. She is also extremely strong willed, and has no problems telling people when they are being idiots. Katsumi has been Ryo's closest friend since early childhood, and secretly loves him. Unknown to Katsumi herself, she is also an Egrigori experiment with a vital part to play in Egrigori's master plan. Katsumi was, in fact, one of the original ARMS-compatible embryos which is why she and Kei bear such a close resemblance to one another. However, she was not given an ARMS implant for unknown reasons. After Ryo's ARMS activates, Katsumi is always beside Ryo trying to keep him calm and sane. Katsumi is taken prisoner by Egrigori in episode 11, but despite her long imprisonment she is always certain that Ryo will come for her. As of volume 18, she has seemingly been killed by the actions of Keith White and Ryo. White used Keith Green's time space jump to pull her into the path of one of Ryo's attacks. Absorbed by Alice, she is then seen alive and well when Black Alice is apparently defeated, but she is the only one to see and offer a hand to her as she crumbles to death. This brief contact leads to the implant of an ARMS core inside Katsumi's body: signs of its growing power are shown on many occasions, in the form of grisly nightmares and increasingly unstable behaviour, culminating in the massacre of a group of thugs. Katsumi's ARMS are nicknamed "Bandersnatch", and can be viewed as the Jabberwock's nemesis. At full power she transforms into an exact copy of Jabberwock, snow-white in color and capable to generate ice, as opposed to Ryo's fire: its power is exceptional, as opposed to the Jabberwock's ultimate ability of anti-matter production and flame manipulation, the Bandersnatch can freeze a town-sized area to absolute zero and create whiteouts with a mere thought.

(Source: Wikipedia)