Kaimu is a sixteen-year-old Sibylla, a priestess-pilot of a Simoun, and is a member of the elite squadron known as Chor Tempest. She is a citizen of the advanced nation of Simulacrum and the older twin sister of Alti. In her role as a Sibylla, she serves as the Sagitta, the navigator who steers their aerial ship, and is the consistent piloting partner of the more assertive Paraietta. Kaimu is physically characterized by her orange hair, which she wears cut to neck-length, and her large, round glasses.

In terms of personality, Kaimu is generally a diffident and timid individual, especially when compared to her more focused and wise twin sister, Alti. Despite her shyness, she is by no means weak-willed; she is alert, easily roused, and capable of displaying significant anger. Her personality is heavily shaped by a traumatic and intimate incident from her past. It is revealed that during a thunderstorm, she and her sister Alti had a sexual encounter with each other. In the aftermath, Kaimu reacted with profound shame and rage, directing her anger entirely at Alti. She blames her sister for the incident and becomes enraged by Alti's very existence, leading to a deep estrangement that lasts for most of the story. This hostility is so pronounced that she finds the very idea of piloting a Simoun with Alti to be unpleasant and refuses to do so for a significant period.

Her motivations and key relationships are the primary drivers of her character arc. Her most prominent motivation is her deep, unrequited love for her flying partner, Paraietta. Kaimu is drawn to Paraietta's leadership, strength of character, and confidence. She expresses her affection in numerous small, caring ways, such as painting Paraietta's nails, making her coffee, teasing her, and simply trying to be near her. Kaimu is very physically affectionate with Paraietta, often kissing her before missions, and feels a great deal of jealousy towards Neviril, another member of Chor Tempest, for whom Paraietta holds deep, longstanding feelings. This unrequited love forms a central part of her identity and her actions within the squadron.

Kaimu's relationship with her twin sister, Alti, is the source of the story's primary conflict for her character. For the majority of the series, Kaimu actively avoids Alti, causing her sister great despair. An episode that forces them to pilot together becomes a breaking point, where the tension between them is fully confronted. During this time, it becomes clear that Kaimu's hatred is a mask for her own guilt and shame, as she privately acknowledges that she was the one who led Alti on that fateful night. Despite her anger, a part of her still remembers a time when she was the protective older sister who would hold the crying Alti and feel stronger for doing so.

This internal conflict leads to significant character development. By the end of Simoun, Kaimu is able to reconcile with Alti. They put aside their traumatic past, realizing that their bond as sisters is what ultimately matters most. Kaimu also demonstrates a notable ability to be deeply caring and consoling, particularly towards Paraietta, and shows herself to be perceptive enough to understand the complex emotional dynamics within Chor Tempest, such as reproaching Neviril for shutting herself away from others. In the series' conclusion, Kaimu makes the definitive choice to remain female at the Spring of Eternity. She and her reconciled sister Alti then return to their childhood mansion, where they choose to live out the rest of their lives together in peace and quiet.