Hikaru Kudou is the younger sister of the protagonist, Kai Kudou, and serves as a central figure in the narrative of E's Otherwise primarily through her fragile condition and the hidden potential she possesses. She is depicted as a young girl with green eyes and green hair that falls to her chest, and one of her notable visual traits includes blunt bangs. Throughout the series, she is frequently seen in a hospital setting, which underscores her role as a character defined by illness and vulnerability.

In terms of personality, Hikaru is portrayed as a gentle and innocent child. Her life has been largely confined to the facilities of the ASHURUM corporation, having never known a life outside its walls. This sheltered existence contributes to her quiet and accepting demeanor. Despite her serious medical condition, she maintains a close and loving bond with her brother, Kai, who regularly visits her. The leader of ASHURUM, Eiji Sagimiya, also takes a particular interest in her, often spending time by her bedside, a gesture that initially seems kind but carries deeper implications.

Hikaru's primary role in the story is that of a motivator and a pawn. Her chronic illness and the promise of advanced medical care provided by the ASHURUM corporation serve as the primary reason Kai agrees to join their special forces unit, AESES. He believes he is indebted to the organization for caring for his sister, and his desire to protect her and ensure her treatment continues drives his early actions. However, the narrative reveals that her significance to ASHURUM extends far beyond that of a sickly patient. It is implied that Hikaru possesses extraordinarily powerful psychic abilities, which are suppressed or inaccessible due to her poor health. This hidden power is the true reason for Eiji's deep interest in her, as she is considered a potentially invaluable asset.

As a result, Hikaru's character development is less about personal growth and more about her shifting role in the plot. Initially presented simply as Kai's beloved, sickly sister, she gradually becomes the story's central trophy or secret weapon. Her condition does not improve despite ASHURUM's advanced technology, and she remains a constant source of anxiety for Kai, who suspects she is being used as a hostage to ensure his compliance. The mystery surrounding her true nature and the potential awakening of her powers represents a significant underlying threat throughout the series.

Regarding her abilities, while she is rarely shown actively using her powers due to her frail state, she is repeatedly identified as possessing psychic potential that is said to far outweigh that of her brother and most other psychics. This untapped, immense power is the core of her value to ASHURUM and the primary danger she represents in the story. For most of the series, her notable ability is not an active one but rather a passive, latent force that other characters seek to control or exploit.