Manaka Kisaragi is a primary character in the anime Boogiepop Phantom. Her origin is deeply tied to the Towa Organization, an enigmatic group involved in human experimentation. Before her birth, her mother, Mayumi Kisaragi, was given a special drug by the organization, and Manaka herself was also administered the Towa drug while still an unborn child by a doctor named Kisugi. This drug caused her abilities to develop from a very early stage. She was born with a condition that led to accelerated aging, meaning that although she appears to be around fifteen or sixteen years old, her actual age is only five.

Her early life was marked by tragedy and isolation. Known as the devil's child, she was killed by her own dying grandmother, who believed she needed to be stopped. However, the alien entity Echoes, during his final sacrifice when he transformed into a pillar of light, revived Manaka. This light not only brought her back to life but also accelerated her aging further and granted her a portion of Echoes power, elevating her to a higher stage of human evolution. Her grandmother died shortly after the revival.

Manaka possesses the supernatural ability to draw out and manipulate the memories of other people, manifesting them as glowing butterflies of light. This power is deeply connected to human consciousness and can be used to trap individuals within their most cherished memories, effectively leaving their physical bodies in a catatonic or phantom state. She can also create other phantom entities, such as Poom Poom, which is born from the memory of a Pied Piper school play and serves as her agent to lure people into her dream world.

Her personality is characterized by a pure, innocent, and naive outlook. She appears unable to speak verbally, and her emotions and intentions are conveyed through her actions and the butterflies she creates. She desires to save people from pain and suffering by returning them to their happiest moments, but her method is ultimately destructive because it stops time and prevents growth. She does not understand that pain and progress are necessary parts of life. This well-meaning but misguided goal makes her the central antagonist of the series, as her actions threaten to trap the entire city in an eternal, unchanging dream.

Manaka shares a profound connection with the Boogiepop Phantom, an entity born from the electromagnetic residue of Echoes light. The Phantom sees her as a kindred spirit, and together they represent the danger of clinging to the past. She is also deeply attached to the memory of her grandmother, who cared for her during her rapid aging, and this bond influences her desire to preserve happy moments forever.

Her role in the story culminates in a direct confrontation with the true Boogiepop, the entity that hunts threats to the world. During this conflict, Boogiepop challenges her simplistic worldview, and Manaka eventually comes to a realization about the necessity of pain, loss, and moving forward. This marks her development from a being trapped in a static desire to a character who understands the value of change, allowing Boogiepop to protect her from the Phantom after her power is lost. Manaka Kisaragi is a pivotal figure whose abilities and motivations drive the thematic exploration of memory, escapism, and the cost of clinging to the past.