Tomoko Arikura is Nanako Misonoo's childhood friend, standing out as the most normal and grounded figure within her prestigious all-girls high school's dramatic environment. She attends the same academy as Nanako and initially shares a close, supportive bond. This friendship fractures when Mariko Shinobu manipulates Tomoko into believing Nanako abandoned her for the elite Sorority. In the manga, this leads to prolonged estrangement, resolved only near the story's end when Tomoko recognizes the misunderstanding and reconciles. The anime diverges, depicting Tomoko overcoming the rift quickly after brief conflict. She remains Nanako's steadfast ally, actively defending her against bullying instigated by Aya Misaki and others envious of Nanako's Sorority status. This includes physical intervention and verbal confrontations, notably using strong language towards Aya's group after they insult Mariko.

Within the anime narrative, Tomoko develops an unexpected friendship with Mariko Shinobu despite their initial conflict. She becomes one of Mariko's defenders against criticism of her background or actions. This dynamic evolves into a trio of close friends with Nanako and Mariko for the remainder of the series. Tomoko provides consistent comedic relief, contrasting the intense emotional turmoil surrounding the Sorority and the "Magnificent Three." Her practical outlook and focus on everyday concerns offer narrative balance against the heightened melodrama of others.

Physically, Tomoko has brown eyes and brown hair typically styled in a ponytail with blunt bangs, often accessorized with a hair ribbon. Her clothing choices favor practicality over Sorority refinement, reflecting her down-to-earth personality. Her athleticism and straightforward demeanor show tomboyish traits, while she also possesses domestic skills like Nanako, sharing proficiency in cooking. Her name, combining the characters for "wisdom, intellect" (智) and "child" (子), and her surname meaning "exist" (有) and "treasure house" (倉), linguistically reinforce character aspects.

Throughout the series, Tomoko maintains a consistent moral compass. Her loyalty proves unwavering, evident in her protection of Nanako against social persecution and her eventual acceptance of Mariko. She embodies enduring friendship and emotional resilience, functioning as an anchor to everyday reality amid the academy's exaggerated social hierarchies and intense personal dramas.