Yoshino Naganohara, Mio Naganohara's older sister, specializes in elaborate pranks targeting her sibling, frequently disrupting Mio's routine. One notable incident involved Yoshino secretly removing batteries from Mio's alarm clock to ensure she overslept, then pursuing the tardy Mio while wearing a bear mask. She later revealed herself first with a luchador mask and then undisguised. When Yoshino tried appeasing Mio by offering ramen bought with money Mio gave her during the chase, it only worsened the situation.

Her personality displays a persistent, mischievous sense of humor, often employing wordplay like punning about "sometimes being a bear" during her antics. Despite Mio's angry reactions and physical retaliation, Yoshino continues her teasing without malice. A flashback shows she once gave Mio the distinctive wooden cube hair clips Mio wears, hinting at a closer past relationship.

Yoshino shares a friendly connection with Misato Tachibana through their mutual attendance at kendo class and sparring sessions, integrating her into the school's social circle. She consistently serves as a recurring source of comedic conflict with Mio, maintaining her established prankster role.