Eru Aharen is a supporting character in the anime Aharen-san wa Hakarenai, introduced as the younger sister of the main protagonist, Reina Aharen. Despite being three years younger than Reina, Eru possesses a physical maturity that often makes her appear older than her sister. She is tall for her age, with long silver hair styled similarly to Reina, and sharp, penetrating eyes that have been compared to those of a snake. Her figure is also more developed than her older sister's, and she is typically seen wearing her middle school uniform, which consists of a white sailor outfit with a navy blue skirt.

Eru's personality is a classic example of a tsundere archetype. Outwardly, she presents a cold, severe, and sometimes confrontational demeanor, especially towards those she perceives as a potential threat to her family. This is particularly evident in her first encounters with Shota Raido, Reina's close friend, whom she views with initial hostility and scrutinizes carefully. However, this harsh exterior is a shield for a deeply caring and affectionate nature. Behind the scenes, Eru shows immense love and protectiveness for her older sister, frequently performing thoughtful acts of service without seeking recognition or praise. For instance, when Reina fell ill, Eru was the one who prepared her meals and ensured her comfort. She blushes easily when complimented, and her genuine wish is for Reina's happiness.

Eru's primary motivation is the well-being of her family, particularly her sister Reina. She takes her role as a caretaker seriously, often feeling a sense of responsibility that pushes her to be more mature than her age would suggest. This protective instinct extends to being wary of Shota Raido. While she is stern with him due to his close relationship with Reina, she is not truly opposed to their bond. Her feelings towards Raido are complex, encompassing distrust, rivalry, and a grudging respect. She later admits to Reina that she finds him interesting and does not dislike him, showing that her initial hostility may stem more from overprotectiveness than genuine disapproval. Eru also has a younger brother named Ren Aharen, and the three siblings share a close, if sometimes quirky, bond.

In the story, Eru serves to add depth to the main character's family life and provide a new dynamic for Raido to navigate. Her introduction in the anime occurs when Raido visits the Aharen household to deliver school printouts to the sick Reina. During this visit, Eru confronts him, questioning whether he is worthy of being friends with her sister. Despite her sharp attitude, she shows her caring side by bringing home an entire tuna and expertly carving it with what appears to be a katana, intending to prepare a meal for the family. Her role in the narrative often involves her observing and reacting to the evolving, ambiguous relationship between Reina and Raido, as she is perceptive enough to suspect that Reina harbors special feelings for him. Over time, through the connection between their older siblings, Eru also begins to interact more with Raido's younger sister, developing a friendship that helps her grow and form new bonds outside her immediate family. A notable quirk of Eru is her unique speech pattern, as she often ends her sentences with the peculiar phrase "desu shi," which contributes to her charm and distinguishes her from her older sister.