Haruka Kaminogi is a twelve-year-old girl who lives in Hakodate with her divorced mother, Asuka Kaminogi. Her father, Takuya Mayuzumi, is a scientist working in the field of quantum physics, and his absence is a notable part of her everyday life. Since she was a young child, Haruka has possessed the ability to see strange phenomena, which she initially believed were ghosts. In truth, these visions were glimpses of other children from alternate dimensions, a sign of her unique and crucial nature.

In terms of personality, Haruka is a grounded and believable character who displays a realistic inner strength and selflessness. Despite the emotional weight of her rough upbringing and the immense pressure she comes under, she remains compassionate and determined. She is curious and possesses a quiet ferocity and concern for those she cares about, which makes her a compelling and relatable protagonist. Her strength is not defined by physical force but by her resilience and moral clarity in the face of terrifying and confusing events. She is frequently pulled into conflict, such as when warriors from another dimension known as the Dragon Knights arrive in her timeline to seek her out, but she faces these extraordinary circumstances with a down-to-earth attitude.

Haruka’s primary motivation throughout the story stems from her deep care for her friends, particularly her childhood friend Yuu. Her role is that of the Ultimate Observer, a being with the extraordinary power to decide what is real and what is not across the entire multiverse. This power is tied to an entity known as the Dragon Torc, which manifests as a necklace and activates when she is in life-threatening danger. As the Ultimate Observer, she controls quantum probability, which grants her the ability to change reality, rewrite entire universes, and even determine which beings have the right to exist. For example, she demonstrates this power by rejecting the interdimensional being Noein from her plane of existence and by swapping objects between different universes. She exists in a state of quantum uncertainty, making her neither truly alive nor dead, and her consciousness resides in the dimension of La'cryma, a world of pure thought.

The most significant relationship in Haruka's life is with Yuu, a troubled and sometimes withdrawn boy. Their connection is the emotional heart of the narrative. The plot further complicates this bond by introducing Karasu, a warrior from the future who is revealed to be an adult version of Yuu. Haruka is forced to confront the ways people can change over time, as Karasu is a no-nonsense soldier, while the young Yuu is still uncertain and struggling with his own familial conflicts. This dynamic between the present Yuu and his future self serves as a powerful driver for Haruka's own development and her understanding of possibility and choice. She also has meaningful interactions with her group of school friends and with the various factions from the future who view her more as a tool called the Dragon Torque than as a person.

Throughout the series, Haruka undergoes significant development as she comes to terms with her role. Initially, she has no control over her immense powers and is often confused by the events unfolding around her. Her journey involves learning to master her ability to perceive and stabilize reality across multiple states at once. By the end of the story, she grows into a decisive figure who actively uses her will to shape the multiverse. In a final demonstration of her authority as the Ultimate Observer, she denies the very existence of the antagonist Noein, effectively erasing him and all his parallel versions from reality. This act showcases her transformation from a scared young girl into a being capable of rewriting the fundamental laws of causality and existence. Her notable abilities include reality warping, probability manipulation, existence erasure, dimensional teleportation, and the passive creation of a barrier through the Dragon Torc that reflects any attack.