Aki Nijo serves as the school nurse at Tenbi Academy and also acts as the staff advisor for the Student Council Security Committee, a group better known as the Maken-Ki. Her connection to the organization runs deep, as she was a member of the very first Maken-Ki during her own time as a student at the academy, alongside her former classmates Minori Rokujo, Tomika Amato, Gen Tagayashi, and Akaya Kodai.

Aki is a tall, fair-skinned young woman with an extremely curvaceous figure, characterized by a very large bust, a small waist, and wide hips. She has waist-length blue hair, which appears a slightly darker or more purple shade in the second season, and light brown eyes that similarly shift to purple. Her typical attire is quite revealing, consisting of a long white lab coat worn open over a low-cut pink or blue-gray top that shows a significant amount of cleavage, paired with a very short skirt and high heels. A constant accessory is a teal or blue eye-shaped pendant, which is the physical form of her Maken, Valhelm. A running visual gag throughout the series involves her unintentionally exposing her underwear, usually when she bends over or stretches, highlighting her naive immodesty.

In terms of personality, Aki is portrayed as one of the most benevolent and gentle figures at Tenbi Academy. She is genuinely kind to both students and staff, taking her duties as a school nurse seriously by providing medical treatment, instructing classes, and even helping students like Takeru Ooyama with their training. Despite her mature and voluptuous appearance, she is surprisingly naive and is a self-described clothing klutz, often finding herself in embarrassing situations without seeming to realize the immodesty of her actions. Underneath her composed and professional exterior, she harbors personal insecurities, most notably the fact that she has never been in a relationship and is still a virgin, a fact she is very sensitive about and becomes mortified when it is accidentally revealed.

Aki's primary motivation stems from her caring nature, wanting to look after the well-being of the students at Tenbi, especially the members of the Maken-Ki. She is dedicated to her work and to maintaining peace at the academy. On a personal level, she desires a romantic relationship but wishes to find a partner on her own terms, which puts her at odds with her mother, who frequently tries to arrange marriages for her. Her role in the story is multifaceted. As the school nurse, she is a source of healing and support, often patching up characters after battles. As the faculty advisor, she provides guidance and training, particularly to the protagonist Takeru Ooyama, whom she mentors alongside Minori Rokujo. She also acts as a bridge between the student members of Maken-Ki and the faculty, and her history as a first-generation member makes her a key figure in understanding the organization's past.

Her key relationships heavily define her character. She shares a deep, long-standing friendship with Minori Rokujo, the principal of Tenbi and founder of Maken-Ki, dating back to their own school days. The most significant relationship in the series is her evolving dynamic with Takeru Ooyama. What begins as a standard teacher-student relationship gradually deepens into romantic affection. To avoid an arranged marriage, she sends her mother a photo claiming Takeru as her boyfriend, which leads to the two being forced to spend time alone together on a trip, where they grow much closer. During this time, Takeru's sincere compliments about her being a wonderful person and a good bride boost her self-esteem, and she later comes to identify him as her first love. She also has a complex history with fellow first-generation member Akaya Kodai, initially helping to recruit him into Maken-Ki and later clashing with him over his morally ambiguous actions.

Throughout the series, Aki undergoes subtle but meaningful development. She begins as a somewhat insecure woman who is self-conscious about her lack of romantic experience. Through her interactions and growing bond with Takeru, she gains confidence in her own worth as a person and a potential partner, moving past her earlier doubts about being a good wife. She remains a consistent source of compassion and strength, even when facing dangerous situations, such as being captured and brainwashed by the antagonist Takeru Yamato to serve as a sacrifice for breaking a seal, from which she is ultimately rescued.

Aki's notable abilities stem from her control over Element and her unique Maken, Valhelm. Valhelm is a debugger-type Maken that takes the form of her eye-shaped necklace. Its primary function allows her to perform a medical diagnosis on a person, visually perceiving damage and injuries within their body as black spots, with darker areas indicating more severe trauma. This makes her an exceptionally effective healer. In addition to its diagnostic capabilities, Valhelm also grants her the power to erase short-term memories, a skill she has used in comedic situations, such as making Takeru forget an accidental glimpse of her underwear. Beyond her Maken, Aki is proficient in using her Element for various spell techniques, including combat, midair levitation, and sealing spells, demonstrating that she is a capable fighter in addition to being a medical professional.