The culprit is Kaori Kosuda. She stole the ticket from Haruka Kona because she wanted to impress her boyfriend, a guitarist, on his birthday. However, at the day the ticket was stolen, she overheard Kona telling her friend that her own boyfriend was also a guitarist and would have his birthday at exactly the same date, indicating that her man was really an exploitative two-timer. Therefore Kosuda committed the theft to draw his full and undivided attention to her.
After stealing it, Kosuda hid the ticket in the pocket of her cafe apron. However, after her shift at the cafe had ended, Masako Moka mistakenly took Kosuda's apron with her for wahing, thinking it was hers. When Kosuda noticed the loss and deduced that Moka had it, she followed her home, waiting for her to leave so she could retrieve the ticket from her apron, which was already in the washing machine. However, Moka returned unexpectedly early and caught her in the act. She had found the ticket before she put the apron in the machine, and was about to sell it on the Internet to save Kosuda from her two-timing boyfriend. As the two argued, Kosuda pushed Moka too hard, which resulted in the latter receiving her fatal head injury.
Kosuda attempted to make the accident look like the work of a burglar, and in order to faciliate the ruse, she replaced her own appron with Moka's. However, Moka had overlooked a red ball pen (used in the cafe for writing down customer orders) in the pocket of Kosuda's apron, and as the apron was being washed, the pen's ink was diluted and spilled out, staining Moka's other laundry pieces in the machine. While the cafe's aprons are uniformingly black, the apron's white care label was stained pink by the ink, thus proving Kosuda's guilt.
Later on, while discussing the guitarist "boyfriend's" guilt in this tragic case, a surprised Conan, Ran and Sonoko incidentally witness Haruka violently cutting her ties with the cheater, giving him his well-deserved dues.
