Haruna Otonashi is introduced as a first-year student at Raimon Junior High who initially works for the school's newspaper club. Her background includes a significant personal tragedy; when she was five years old and her brother Yuuto was six, their parents died in a plane crash. The two siblings lived in an orphanage before being adopted by different families, a separation that created an emotional rift between them for several years. She was taken in by the Otonashi family while her brother was adopted by the Kidou family. This backstory deeply influences her character and her relationships with others throughout the series. In her first appearance, she approaches Endou Mamoru to write an article about the soccer club's upcoming match against the powerful Teikoku Gakuen. Impressed by the team's fighting spirit and dedication, she decides to become the second manager of the Raimon soccer team, working alongside Kino Aki. She is the biological younger sister of Kidou Yuuto, the genius playmaker initially serving as the captain of Teikoku Gakuen. This connection is a major emotional core of her character. After years of separation and a lack of contact from her brother, she confronts him with her feelings of hurt and abandonment. Their relationship is repaired when Kidou is freed from the manipulative influence of Kageyama Reiji and reaffirms that he never forgot her. In the later series Inazuma Eleven GO, Haruna has grown up and becomes a teacher at Raimon Junior High as well as the advisor and tactical consultant for the new generation of the Raimon soccer club.

Haruna has a vibrant and energetic personality that often contradicts the meaning of her surname, Otonashi, which can imply silence. She is loud, talkative, and intensely curious, with a strong passion for investigating and gathering information, a skill she honed in the newspaper club. Her enthusiasm can lead her to take extreme measures, such as wearing a mustache as part of a disguise. Despite this energetic exterior, she is also deeply kind-hearted, caring, and responsible. She performs her duties as a manager with diligence, helping with everything from logistical support to preparing snacks for the team. Her own experience with loss and separation gives her a profound sense of empathy. This is most clearly demonstrated in her relationship with Kogure Yuuya, a player who is distrustful of others due to feeling abandoned. Haruna understands his pain, takes on a patient, motherly role, and successfully helps him learn to trust and open up to his teammates. As an adult in Inazuma Eleven GO, her personality is more mature and calm, though she retains her dedication to the team and her keen observational skills. She is shown to be a supportive and knowledgeable advisor, though she can still express frustration, particularly when she misunderstands her brother's strict coaching methods as being unnecessarily harsh.

Haruna's primary motivation stems from her love for soccer and the spirit of the players. Her initial decision to become manager was fueled by the inspiration she felt watching Raimon play. She is driven to support the team in any way she can, primarily through her exceptional ability to collect and analyze data on opposing teams. On a personal level, she is motivated by a desire to reconnect with and support her estranged brother, Kidou. Her arc in the original series heavily revolves around resolving the emotional distance between them and solidifying their bond. In the series, her role evolves significantly. She starts as a journalism student and becomes a key support member for Raimon. During the Aliea Academy incident, her emotional intelligence and persistence are crucial in reforming Kogure and integrating him into the team. During the Football Frontier International tournament, she continues her duties as a manager for Inazuma Japan. She also becomes a target, being hypnotized and kidnapped by the demonic team Makai Gundan Z as a sacrifice, an event that directly motivates her brother to lead a rescue mission. In Inazuma Eleven GO, her role shifts from a peer supporter to an authoritative yet caring faculty advisor, guiding a new generation of players, most notably Matsukaze Tenma.

Her key relationship is with her older brother, Kidou Yuuto. Their bond, marked by separation, misunderstanding, and eventual reconciliation, is a significant emotional storyline. Another important relationship is with Kogure Yuuya, where she acts as a maternal figure who sees past his troublesome pranks to his underlying pain. She also shares close friendships with her fellow managers, Aki and Natsumi, often hugging them in moments of high emotion. As an advisor in the GO series, she has a professional and supportive relationship with the new Raimon players and shows great joy and relief when Endou Mamoru returns as the team's coach.

Haruna's notable abilities are not physical soccer techniques used in matches, but rather intellectual and supportive skills. Her greatest asset is her mastery of information gathering and tactical analysis. She can uncover detailed data on opposing teams, including rumors, formations, and player weaknesses, which provides a strategic advantage. She is also capable of understanding and articulating complex tactical concepts, making her an effective adviser in her later role as a teacher. In some video game adaptations, she can be recruited as a player, where her listed abilities include support-oriented moves such as Lucky!, Super Scan, Assist!, and Ribbon Shower. Her character development is visible from a lively, talkative middle schooler dealing with family trauma to a composed, reliable, and respected teacher who uses her sharp mind to nurture future soccer stars.