Aiko Senoo is a central character known for her energetic and straightforward personality. She is a transfer student who moved from Osaka to the city where the main story takes place, and she proudly speaks in the Kansai-ben dialect, which often makes her stand out among her peers. Her appearance is distinct, characterized by her short blue hair styled into split curls and her blue eyes. Her casual outfits typically consist of practical clothing like blue overalls with a green shirt in the earlier parts of the series, later replaced by blue pants and a long-sleeved green shirt, reflecting her active lifestyle.

Aiko serves as the quintessential tomboy of her friend group. She is highly athletic and dynamic, boasting physical prowess that surpasses most boys in her class, with the notable exception of swimming, which is a personal challenge for her. Her temperament is passionate and she has a quick temper, but this is balanced by a deeply caring and loyal nature. She is often the one to cheer up her friends during difficult times, using her humor and pragmatism to provide support. Despite her tough exterior, she has vulnerable moments, particularly concerning her family. She treasures a harmonica that her parents bought for her when she was younger, and she is highly protective of it even after it becomes damaged.

The central motivation in Aiko's life is her desire to see her divorced parents reconcile. Her father works as a busy taxi driver in the city where she lives, while her mother works as a nurse back in Osaka. This separation weighs heavily on her, and she is known to go to great lengths, even attempting forbidden magic, in hopes of reuniting them. This personal struggle shapes much of her emotional development and her actions throughout the story. Her character arc culminates in her wish being fulfilled, and she eventually returns to Osaka to live happily with her parents and grandfather.

As a witch apprentice, Aiko is associated with the color blue. Her magical instrument is a harmonica, which matches her real-life skill with the instrument, and her accompanying fairy partner is named Mimi. Her magical abilities and spells often reflect her strong and determined character, focusing on themes of unity and strength. Her role within her friend group is that of a pragmatic and blunt yet steadfast pillar, someone who is not afraid to speak her mind or mediate conflicts, such as the tensions between other members like Onpu and Momoko. She has a strong sense of responsibility, demonstrated when she helps care for Hana, the future Witch Queen.

Aiko's key relationships extend beyond her immediate family and magical companions. She shares a deep bond with her fellow witches, Doremi Harukaze and Hazuki Fujiwara, who helped her become an apprentice. She is a loyal and protective friend, often using playful teasing to show her affection and pushing her friends like Hazuki to be more confident. She has a notable rivalry with Leon, a member of the male idol group FLAT4, who constantly challenges her to sports matches; she almost always wins, and despite finding him annoying, she begrudgingly respects his competitive spirit. Aiko is also friends with Nobuko Yokokawa, a classmate who initially lied to gain Aiko's admiration; after the misunderstanding is resolved, Aiko becomes a supportive friend who looks forward to Nobuko's stories. A specific storyline in Ojamajo Doremi Na-i-sho involves a childhood friend named Keiichi Harima, who reappears to remind her of a childhood promise to marry him, a promise she made to hide her inability to swim. While she does not share his romantic feelings, her care for him is evident.

Aiko undergoes significant personal growth, particularly in confronting deep-seated vulnerabilities. One such instance involves a childhood fear of older male authority figures, stemming from a memory of violence involving her grandfather. This fear is tested and begins to heal during a visit to Doremi's grandparents' home, where she learns to build trust and overcome her anxiety. Her development is a journey of learning to untangle the knots of her past while using her candid and unwavering support to anchor her friendships. Her straightforward nature is reflected in her practical approach to magic and problem-solving, as well as her particular talents outside of athletics, such as her skill in making takoyaki. She holds such strong opinions on the proper preparation of certain foods like baked sweet potatoes, hot pot, and curry that her friends call her a "奉行" (bugyō), a commissioner or enforcer of rules.