12-Sai. Koisuru Diary (Japanese: 12歳。恋するダイアリー) is a romance simulation game developed and published by Broccoli for the Nintendo 3DS. The game is based on the manga series 12-Sai. Koisuru Diary, written by Nao Maita, and follows the story of a 12-year-old girl as she navigates her first love. Players take on the role of the protagonist and experience her story through a series of conversations, decisions, and events. The game features a cast of characters from the manga, including the protagonist's love interest, as well as a variety of minigames. Players can also customize the protagonist's appearance and clothing, as well as her diary. 12-Sai. Koisuru Diary is a lighthearted and heartwarming story about young love, and is sure to be a hit with fans of the manga series.