Kuroe Akaishi is the protagonist of the story, a high school girl whose life is defined by a secret and unusual condition. For as long as she can remember, she has suffered from a mysterious illness that causes her body to undergo abnormal growths and disfigurements. This affliction is directly tied to her emotional state, specifically triggered by intense feelings of love, embarrassment, or excitement; when these emotions become overwhelming, she physically transforms into a colossal kaijuu, a giant monster. Because of this, Kuroe has grown up as an introverted, quiet, and isolated outcast, having earned the cruel nickname "Psycho-tan" from her classmates who do not understand her condition. Her deepest fear is that no one could ever accept her for what she truly is, leading her to believe she is destined to be alone and to try her best to live a normal, unremarkable life in the present.

Her personality is a mix of deep-seated insecurity and a quiet, sweet nature. She is often a bundle of nerves, prone to self-hatred and pushing people away out of fear of hurting them or being rejected. Despite this, she longs for connection and happiness, which creates a powerful internal conflict. Her primary motivation throughout the story is to navigate her budding romantic feelings for her classmate, Arata Minami, while desperately trying to hide her monstrous transformations from him. This becomes her central struggle as she grapples with the possibility that a bright and popular boy could truly care for an outcast like her.

Kuroe’s role in the story is driven by her internal battle, but her actions have external consequences. When her emotions peak and she transforms, her kaijuu form often rampages through Tokyo, becoming a looming threat that the public and media must contend with. Her personal drama and the city's disaster response are thus inextricably linked. Her key relationships are few but transformative. The most significant is with Arata Minami, a kind and popular boy who takes a genuine interest in her, showing her the possibility of love and acceptance. Her devoted mother, Rinko Akaishi, is a former biologist who knows Kuroe's true nature as a kaijuu and does everything she can to protect her and make her feel loved, even if her methods are sometimes eccentric. Kuroe also forms a close friendship with Manatsu Tomosato, a wealthy and eccentric girl at her school who is unknowingly obsessed with Kuroe's kaijuu form.

Over the course of the narrative, Kuroe experiences significant character development. While initially defined by her loneliness and lack of self-worth, her relationship with Arata pushes her to want to gain confidence in herself, open up to others, and learn to communicate better. Though she still struggles with her condition, she begins to find value in her connections and even tries to use her monstrous abilities for good, such as helping a friend in need. Her journey is a central metaphor for the awkwardness, anxiety, and intense emotions of adolescent romance. In terms of notable abilities, Kuroe possesses the power to transform into a giant kaijuu when her emotions, particularly those related to love, become too intense. This transformation is not a fixed state; she can initially sprout reptilian features such as scales or a tail as her embarrassment grows, leading to a full monstrous metamorphosis when her feelings peak. Her true origin, revealed by her mother, is that she is not a human with an illness, but a kaijuu who was raised as a human child.