Hikari Horaki is a thirteen-year-old student and the class representative of Class 2-A at the municipal school in Tokyo-3, the same class attended by the primary Evangelion pilots Shinji Ikari, Asuka Langley Soryu, and Rei Ayanami. She is responsible and takes her duties seriously, often ensuring that her classmates follow proper protocol and leading them in ceremonial greetings for the teacher. Her name, along with those of her older sister Kodama and younger sister Nozomi, is derived from the three services of the Tōkaidō Shinkansen bullet train line. She lives with her two sisters and is implied to be the primary caretaker for the family, as her mother is deceased and her father is largely absent, often away on business. This responsibility has made her an accomplished cook.

Initially, Hikari may come across as slightly aggressive, rigid, and no-nonsense due to her role as class representative. She is often seen arguing with her classmate Toji Suzuhara. However, beneath this exterior, she has a warm and understanding personality. She is kind-hearted and surprisingly traditional, as evidenced by her own surprise at her skill in cooking, which she considers a feminine trait at odds with her strict public persona.

One of her most significant roles in the story is being Asuka’s only close friend and confidante. While many of their classmates are put off by Asuka’s abrasive and arrogant behavior, Hikari makes an active effort to understand her and form a bond. She is perceptive about her friend’s feelings, recognizing Asuka’s unspoken affection for Shinji and encouraging a connection between them. In turn, Hikari confides in Asuka about her own personal struggles, most notably her secret crush on Toji Suzuhara. Although she often bickers with Toji, she is deeply fond of him, finding him to be kind at heart. She struggles to confess her feelings directly, instead showing her care through actions, such as offering to make him homemade lunches after learning he only eats store-bought ones. When Toji is chosen to become the pilot of Evangelion Unit-03 and is subsequently grievously injured during its activation, Hikari visits him in the hospital, demonstrating the depth of her concern.

As a member of Class 2-A, Hikari is part of a select group of students who are unknowingly potential candidates to pilot an Evangelion. Like many of her peers, she shares the common background of having lost her mother. While she displays no notable combat or piloting abilities in the main narrative, official artwork has depicted her as a possible pilot for Eva Unit-05. Her strength lies in her emotional resilience and her capacity for empathy, which stands in stark contrast to the troubled psychological states of the main pilots.

Throughout the series, Hikari serves as a grounding presence, representing a semblance of normalcy. Her home life and friendships provide a point of reference for what the pilots, particularly Asuka, are fighting to protect. When Asuka suffers a severe mental breakdown after a traumatic battle, Hikari offers her a place to stay at her house, telling her friend she does not need to explain herself and that she believes she did her best. This act of quiet, non-judgmental support is a pivotal moment in their friendship. Hikari also develops a fondness for Misato Katsuragi’s pet penguin, Pen Pen, and eventually takes him in to live with her family when Tokyo-3 is evacuated. In the story's final episodes, an alternate version of Hikari appears in a parallel reality, still serving as the class representative in a world where the Evangelion pilots live ordinary lives.