Katsumi Akagi, defined by compassion and willpower, is Ryo Takatsuki's childhood friend and neighbor. A deep bond formed when young Ryo consoled her during the burial of a deceased cat. Unbeknownst to her initially, she is one of Egrigori's original ARMS-compatible embryos, sharing a physical resemblance with Kei Kuruma due to their engineered origins; she received no initial ARMS unit implant.

After Ryo activates his ARMS, Katsumi provides steadfast emotional support to maintain his stability. Egrigori later kidnaps her for their schemes. During a confrontation manipulated by Keith White using Keith Green's spacetime abilities, Katsumi is fatally struck by one of Ryo's attacks. Absorbed by Alice, she is revived following Black Alice's defeat. Brief contact with the dying Black Alice results in an ARMS core being implanted into her body.

This implant triggers disturbing nightmares and increasing instability, culminating in violent outbursts. Her latent ARMS unit, Bandersnatch, manifests fully during these episodes. Appearing as a white counterpart to Jabberwock, Bandersnatch wields cryokinetic abilities, generating extreme cold capable of freezing town-sized areas to absolute zero and creating instant whiteouts. The ARMS unit's influence intensifies her aggression, leading to lethal confrontations.

Katsumi's psychological struggles are compounded by Keith Green's fixation on her, though he cannot understand her emotional focus on Ryo. Her relationship with Ryo culminates in marriage, and they have a child. This child is implied to be the reincarnation of the redeemed Black Alice.