Aria Aquarius is a prominent character from the anime The Classroom of a Black Cat and a Witch, serving as a childhood friend, rival, and classmate to the protagonist, Spica Virgo. She is a student at the Royal Diana Academy and is a member of the coven known as Pandora's Box. Her name and abilities are derived from the Aquarius zodiac sign, and she is recognized for her distinctive long, purplish-dark blue hair with lavender strands, pink eyes, and a smirk that often signals her playful intentions.

Aria possesses a confident and teasing personality, frequently taking pleasure in making condescending comments or laughing at the failures of others, especially her friend Spica. This behavior has earned her the description of a bratty girl or a femme fatale. Despite her sharp tongue, her teasing is not meant to be malicious, and she genuinely cares for Spica, becoming worried when her friend is in danger and actively supporting her in battles. Aria is highly confident in her own talents, often calling herself a genius, and this self-assurance extends to many activities, including drawing, cooking, and horse riding. This confidence can sometimes border on recklessness, leading her to confront large monsters on her own.

Beneath her outward bravado, Aria has a more complex motivation rooted in her past. She is an only child who showed early talent in swordsmanship, which led her father, who dreamed of becoming the kingdom's strongest, to push her into intense training. The pressure from this, combined with her parents' separation after she placed second in a competition, caused her to abandon the pursuit of mastery. Instead, she shifted her focus to simply having fun and enjoying herself. Initially, she had no interest in magic but began learning it solely to have something to tease Spica about when Spica declared her dream of becoming a mage. Over time, witnessing Spica's genuine dedication and hard work caused Aria to feel a sense of admiration, and through their friendship, she rediscovered her own desire to improve and pursue her own dreams.

Aria plays a significant role as Spica's primary rival and closest companion at the Royal Diana Academy. During the entrance exam, the two worked together to defeat a golem, a turning point that helped Aria take Spica's ambitions more seriously. Within the story, she acts as a foil to Spica, possessing natural talent and prior knowledge of magic, whereas Spica starts with none. Aria often provides critical support in combat and offers Spica insight when she struggles with magical concepts. She is particularly close to Spica and is also known to show physical and social interest in her other female classmates. She is frequently partnered with her classmate Merrow Pisces, with whom she can perform a powerful Cross Magic combination spell called Aquarium Sphere, which creates a large aquarium-like sphere of water teeming with sea creatures.

Aria's notable abilities center on her proficiency with Aquarius Magic, which allows her to create and control water. She is considered a very talented mage, praised even by her teacher Claude Sirius for her advanced and creative spell usage. Her repertoire includes Hydro Cannon, a powerful blast that can shatter a large golem but consumes a significant amount of her mana. She can also use Aqua Sphere to create a protective watery shield around a target and Hand of Poseidon to form a giant hand of water capable of catching fast-moving targets like Lophorns. Her mastery over her element also extends to freezing the water vapor in the air to create ice. In addition to her magic, she has a background in swordsmanship and is physically fit. Her development throughout the series sees her transform from someone who was content to simply make fun of her friend into a dedicated and talented student who rediscovers her own passion for excellence, driven by the inspiring example of the very person she used to mock.