Ai Mikami is a participant in the survival game as the Seventh diary owner, a role she shares with her partner and lover, Marco Ikusaba. Together, they form a unique pair of holders who wield a shared Future Diary known as the Exchange Diary, which manifests as two mobile phones that predict each other's immediate future. This allows them to coordinate their actions and protect one another with remarkable efficiency, becoming a formidable team. However, this strength becomes a vulnerability if they are separated, as the diary's predictive power is significantly diminished.

Physically, Ai is a young woman with shoulder-length dark blue hair and purple eyes. Her typical attire includes a light blue sleeveless jacket and white pants. Beyond her diary, she is a highly skilled combatant specializing in ranged attacks, using a collection of throwing knives kept in her jacket. She can throw these knives with pinpoint accuracy and blinding speed. While not primarily a brawler like Marco, she possesses above-average close-combat skills, fast reflexes, and knowledge of infiltration and first aid. In addition to her Future Diary, Ai once used an apprentice diary known as the Flirting Diary, granted by her connection to the Eighth, Kamado Ueshita. This secondary diary could identify the men she would flirt with, allowing her to track specific individuals, but it was rendered useless by signal jamming and its destruction would not harm her.

Ai's background is rooted in abandonment. As a child, she was left by her parents at Sakurami Tower and told to wait for them, a day she came to curse as the worst of her life. It was there that Marco approached her, offering her a better tomorrow and inviting her to live at Mother's Orphanage, the home of Kamado Ueshita. This act of kindness forged an unbreakable bond between them, and Ai grew up deeply devoted to Marco. The anime adaptation adds a darker chapter to her past during high school, where she was lured to an abandoned factory by jealous classmates and sexually assaulted. Marco arrived to save her, killing one of her attackers in the process. Consumed by guilt, he later attempted suicide at Sakurami Tower, but Ai stopped him, and they pledged to protect and support each other for the rest of their lives.

Personality-wise, Ai is defined by her devotion to Marco. Their shared goal of becoming the god of a new world is not driven by ambition for power, but by a simple, profound wish to remain together for eternity. Despite the brutal nature of the survival game, she and Marco adhere to a moral code; they treat the wounds of those they injure and refuse to take lives unnecessarily. Ai is generally levelheaded and practical, sometimes serving as a voice of reason compared to Marco's more impulsive nature. However, she harbors deep insecurities about their relationship, and any perceived hint of infidelity can send her into hysterics. She is also implied to be flirtatious by nature, a trait reflected in her apprentice diary.

In the story, Ai first appears working undercover alongside Orin Miyashiro as an attendant to Tsubaki Kasugano, the Sixth, to observe other diary owners. She later confronts the protagonists, Yukiteru Amano and Yuno Gasai, as a direct antagonist. During their first encounter, she and Marco manage to steal Yukiteru and Yuno's diaries and injure Yuno. However, in their final battle at the abandoned Sakurami Tower, the tables are turned. Yuno fatally wounds Ai, and as the tower collapses, a mortally wounded Marco uses his last strength to save Yukiteru and Yuno. In their final moments, Ai and Marco reaffirm their love, having already exchanged vows in the tower's chapel, making them husband and wife. In the reset timeline of the third world, where the survival game does not occur, Ai and Marco survive and are shown living happily together, with Ai appearing pregnant, indicating they have started a family.