Haruna Sairenji is a central figure in the story, known as the girl that the main protagonist, Rito Yuki, has held affections for since their time in junior high school. Her background reveals that she lives with her older sister, Akiho Sairenji, in their family home in Sainan Town, while her parents moved away when she was thirteen due to her father's job. This living arrangement contributes to her sense of responsibility. Her feelings for Rito originated from a specific incident in the past: when the school garden was vandalized, Rito was wrongly accused, but Haruna was the only one who believed in his innocence, a kindness that Rito overheard and which sparked his love for her.

In terms of personality, Haruna is characterized by her kindness, patience, and strong sense of responsibility. She is so highly regarded by her peers that they often vote for her as class representative even when she is not a candidate. She is typically polite and understanding towards everyone, especially Rito, forgiving his frequent and accidental perverted clumsiness almost immediately, though she may occasionally slap him out of reflex from shock or fear. A defining trait is her intense fear of ghosts and supernatural phenomena, which often leads to a comedic reaction where she panics and swings whatever is nearest—frequently Rito—as a weapon to fend off the source of her fright. Despite her own attractiveness, she can feel insecure about her physical development compared to other girls around her.

Haruna's primary motivation throughout the narrative is her romantic love for Rito. However, her motivation is complicated by her deep friendship with Lala Satalin Deviluke, an alien princess who also loves Rito. Haruna initially suppresses her own feelings, believing them to be unrequited and not wanting to hurt Lala by confessing. Her journey involves finding the courage to acknowledge her feelings, eventually confessing them to Lala, who accepts her as a rival while remaining best friends. This relationship dynamic is central to her role in the story, as she represents the down-to-earth, traditional love interest whose progress is constantly interrupted by the chaotic, otherworldly events surrounding Rito and Lala.

Haruna’s key relationships shape much of her character arc. Her bond with Lala is unique; despite being romantic rivals, they share a genuine, supportive friendship. Lala actively encourages Haruna to pursue Rito, introducing the concept that space laws permit polygamy, which would allow them both to marry him. She is also close friends with Risa Momioka and Mio Sawada, though they frequently tease and grope her. Overcoming her fear of ghosts, she forms a close friendship with Oshizu Murasame, who becomes one of the first people to learn of her crush and offers her support. At home, her older sister Akiho often teases her about her relationship with Rito.

Throughout the story, Haruna shows significant development, particularly in the sequel series. While her role is sometimes reduced due to the introduction of many new characters, she gains more personal agency. She moves from passively harboring her feelings to actively trying to resolve them. Under the influence of Deviluke herbs, her repressed desires surface, leading to moments of boldness. In a pivotal event, she and Rito accidentally share a kiss after being shrunken by one of Lala's inventions. Ultimately, she works up the courage to confess her love to Rito directly, finally receiving a confession in return, though Rito asks for time to decide as he also has strong feelings for Lala.

Notable abilities include a surprising, if situational, superhuman strength. When panicked, especially by ghosts, she can lift and swing a person, usually Rito, with immense force. She is also a skilled tennis player, being a member of the school's tennis club. Furthermore, she is left-handed and possesses strong leadership and improvisational skills, such as when she effectively wielded a sword in a virtual reality game by comparing its weight and balance to a tennis racket.