Yumi Miyamoto works as an assistant inspector in the Tokyo Metropolitan Police Department's Traffic Division, handling traffic accident investigations and enforcement duties despite occasionally speeding for personal reasons. She shares a close friendship with Miwako Sato, regularly offering romantic advice about Sato's relationship with Wataru Takagi and playfully encouraging their union. Her romance with Shukichi Haneda started in college after a chance train encounter where he fell asleep on her shoulder. She missed her stop to ensure he awoke at his destination, charmed by his sleeping face. They dated but separated due to misunderstandings surrounding Shukichi's professional shogi ambitions. During their relationship, he gave her a sealed envelope containing a marriage registration form, instructing her not to open it until he won all seven major shogi titles. Mistaking his career dedication for abandonment during his prolonged silence, Yumi ended things and misplaced the envelope. They later reunited and gradually reconciled. Yumi remained unaware of Shukichi's shogi fame until spotting him on television. Learning titleholders earned substantial income, she desperately searched for the lost marriage document. After recovering it with help, she discovered Shukichi had just lost a critical title match, voiding the form's condition. She tore up the document but urged him to keep pursuing all seven titles. Their bond deepened when Shukichi rescued her from a kidnapper mid-Meijin title match; her kiss for luck contributed to his victory. Yumi displays a mischievous, teasing personality, enjoying matchmaking and coaxing male colleagues into buying her gifts. She exhibits romantic possessiveness, having once interfered in Naeko Miike's relationship with Kazunobu Chiba. Despite her confident exterior, she masks vulnerability about her feelings for Shukichi with brusqueness, yet shows steadfast loyalty by supporting him emotionally after shogi losses. In "The Scarlet Bullet," Yumi travels to Nagoya with Shukichi to meet his family. During a date, heavy intoxication leads her to publicly accept a marriage proposal. She expresses a desire for matching couple outfits, confirming their ongoing romance. Shukichi's protective concern emerges when he contacts his brother Shuichi Akai, requesting Yumi's protection during a serial killer investigation targeting traffic policewomen—though Akai notes she appears fully capable.