A second-year student at Millennium Science School and the Treasurer of Seminar. Since the Student Council President, Rio, rarely acts publicly, Yuuka serves as the face of Seminar, handling field command and interpersonal negotiations alongside the Secretary, Noa. She is strong-willed, talkative, and extremely serious about school rules. Due to her position as treasurer, she often clashes with other clubs. To ensure the survival of Millennium, which is always on the brink of bankruptcy, she makes ruthless decisions and will lecture anyone—even the Sensei or Rio—if they lack logic. Because she mercilessly cuts budgets, which are the lifelines of club activities, students fear her under the nickname "The Cold Calculator." In reality, she is a deeply caring person who only makes those budget cuts as a "last resort" after considering the other party's circumstances. Consequently, while some complain, no student truly hates her, and she is actually well-respected. She tends to be soft on her juniors even if they are in the wrong, such as bringing toys to Koyuki while she is in custody. She has a habit of trying to apply calculations to everything and struggles with fields like art, where value fluctuates due to uncertain elements. First person: I (Watashi)

A second-year student at Millennium Science School and the Secretary of Seminar. She is usually thoughtful, calm, and collected. She always wears a composed smile and treats others with polite, cheerful language. She possesses an extraordinary memory, capable of perfectly memorizing information she has seen or heard once. Using this memory and her engineering knowledge to verify functionality just by glancing at blueprints, she acts as a patent attorney managing Millennium's intellectual property. She is shrewd enough to sense danger and avoid it through clever wordplay, and she can hold her own against the school's more eccentric students. Unlike Yuuka, she is well-versed in literature and linguistics. Because of her intimidating aura, her love for pranks, and her habit of using literary expressions (which she calls "indirect language"), she worries that her true feelings don't always reach others. She harbors no ill will toward Koyuki and actually views her fondly, but she finds it regrettable that Koyuki avoids her. She is reportedly poor at physical exercise, frequently losing to vaulting boxes and balance balls. First person: I (Watashi). Generally speaks politely.

A first-year student at Millennium Science School. Her official position in Seminar is unknown. At the start of the story, she had been fired from Seminar and was being held as a criminal under surveillance. Since then, she has repeatedly been put on the wanted list, captured, escaped, and thrown back into the reflection room. She returned to Seminar during the Final Episode due to certain circumstances. She possesses the unique ability to intuitively crack even the most complex computer encryption. She can also intuitively solve physical passcodes, which seems to be a natural talent, though she is entirely unaware of it. Because she is free-spirited and lacks a strong sense of ethics, she causes various problems and is frequently suppressed by C&C. The worst part is that she never learns, no matter how much she is scolded. She has a selfish personality, matter-of-factly stating things like, "If you don't have money, you should just steal it from somewhere." Noa is the one person she absolutely cannot stand up to. According to Koyuki, Noa is "sometimes truly terrifying, enough to be in my dreams" and "remembers exactly what I said down to the minute, and points it out if my story changes." She generally speaks in a polite tone. First person: I (Watashi)