Sophie Senoo, called Aiko Senoo in Japanese-language media, comes from Tengachaya, Osaka. She moved to Misora with her taxi driver father due to his work. Initially brash and outspoken, she speaks with a distinct Kansai dialect she dislikes others copying. Her rough early interactions, like accidentally angering Doremi Harukaze, evolve into a close, teasing friendship.

She has fair skin, dark blue eyes, and short blue hair ending at her shoulders with flared tips. Short bangs expose her forehead, featuring a distinctive curl. Her first outfit is a kiwi-green and yellow shirt with royal blue denim overalls, later changing to a plain green T-shirt, khaki vest, and denim pants. She habitually places her hands in her overalls' bib instead of pockets.

Sporty, bold, and passionate, her bluntness can unintentionally seem rude. Academically challenged but athletically gifted, she excels in most sports except swimming—a skill she never learned due to childhood frustration with her father's teaching attempts. This insecurity prompted her to promise marriage to an old friend, Keiichi Harima, to hide her inability, a pact revisited in the OVA *Na-i-sho*. Beneath her tomboyish exterior lies maturity and responsibility, rooted in her parents' divorce during childhood. After an accident hospitalized her, she overheard her parents arguing and blamed herself, choosing to live with her father believing he needed her support more. This concealed deep emotional pain, particularly her longing for her mother, Atsuko, whom she met secretly. Aiko also carries trauma from witnessing her grandfather's violence toward her mother, fostering distrust of older male relatives.

As the third witch apprentice, her magic color is sky blue, partnered with the fairy Mimi and a diamond-shaped crystal ball. She joined after witnessing Doremi's magic and spying on Majorika, who recruited her and Hazuki Fujiwara to protect Doremi's secret. She mastered broom flight faster than her peers, using transformation incantation "Pretty Witch Aiko-cchi!" and spell phrase "Pameruku Raruku Rarirori Poppun!" Her magical stage invocation is "Pameruku Raruku, Takaraka ni!"

Her development unfolds across seasons. In season one, she discovers her mother's unsent letters, leading to a failed reconciliation attempt after mistakenly believing her mother had a new family. Season two sees her struggle with maternal inexperience while helping raise the witch-infant Hana, bonding deeply and becoming protective and nurturing. Concurrently, she receives a letter from her mother, sparking efforts to reunite her parents. Season three deepens this pursuit, with her father attempting reconciliation for her sake. By season four, her persistence succeeds: her parents remarry, though her grandfather's illness forces a move to Osaka. This requires her to leave her friends and choose between becoming a full witch—gaining longevity but separating from family—or remaining human. She chooses the latter, prioritizing family.

Her relationships reveal her multifaceted nature. With Hazuki, she balances gentleness and stern encouragement. She initially dislikes Onpu Segawa for manipulative tendencies but grows to value their friendship, often noticing when Onpu needs support. She mentors Momoko Asuka, helping her navigate social missteps from cultural gaps. Aiko adores Hana, frequently holding her and learning childcare from Doremi's mother, though she worries about being less nurturing than peers. She admires Nobuko Yokokawa until discovering Nobuko's lies about shared hardships, forgiving her after understanding Nobuko's desire for closeness. Her dynamic with Leon of the FLAT4 involves constant sports challenges she always wins, finding his random English usage irritating.

Aiko plays harmonica and drums, enjoying comedy routines reflecting her Osaka roots. Her name etymology signifies "love child" (Aiko) and "little sister tail" (Senoo), aligning with her role as a protective, familial figure.