Touka Miyashita is a high school student at Shinyo Academy, appearing as a perfectly ordinary and cheerful young woman. She is known for her bright, honest, and mischievous personality, getting along easily with her peers and showing a good-natured willingness to encourage her friends when they are feeling down. Despite her friendly demeanor, she is not particularly academically gifted and often relies on her close friend, Kazuko Suema, for tutoring to pass her exams. In her personal life, she is in a romantic relationship with an upperclassman named Keiji Takeda, for whom she cares deeply.

Unbeknownst to Touka, and even to herself, she is the host of a mysterious, supernatural entity known as Boogiepop. This entity, who sees themselves as a guardian that manifests to fend off "enemies of the world," is an alternate personality that resides within her mind. The two personas are starkly different; where Touka is warm and ordinary, Boogiepop is a logical, emotionless, and androgynous figure. Touka is completely unaware of this dual identity. Whenever Boogiepop takes control to face a threat, Touka loses all memory of the events. Her mind automatically fills these gaps with fabricated, ordinary memories, effectively erasing any evidence of the entity's emergence and leaving her with no suspicion that she is anything other than a normal student. She is often seen carrying a large Spalding sports bag, which unconsciously contains the distinctive cloak and hat that make up the Boogiepop outfit.

The relationship between Touka and Boogiepop is the central paradox of her existence. Boogiepop is aware of Touka and her life, but Touka remains oblivious. This dynamic places her in a unique position within the story: she is both central and peripheral, as her body is the vessel for the main protagonist, yet her own conscious self is a passive participant in the supernatural conflicts. Her role serves as a grounded, human counterbalance to the abstract and cosmic threats that Boogiepop confronts, highlighting themes of duality, identity, and the value of ordinary life. Only a select few characters, including her boyfriend Keiji Takeda and the enigmatic Nagi Kirima, are aware that she and Boogiepop are the same person.

The consequences of being Boogiepop's host are significant and often detrimental to Touka's personal life. The entity's actions do not take her schedule into account, leading to various hardships. The most notable example is when Boogiepop manifested on the day of her university entrance examination, causing her to fail the test and miss her chance to enter higher education—an outcome that even Boogiepop admitted was a mistake. Her family situation has also been strained by her secret; her mother once witnessed Boogiepop and was subsequently treated as insane by the rest of the family, who did not believe her story. Despite the entity's claims of having no emotions, Boogiepop and Touka share a core similarity in their strong desire to help others, suggesting that the entity's sense of justice is rooted in Touka's own nature. She has a small, peculiar trait in that she cannot whistle, which is another subtle distinction between her and her other self.