Originally thought to be two distinct Diary users, Marco and Ai are actually the same Seventh Diary user. Ai initially appeared early in the series as a member of the Onukata cult, working undercover with her friend Orin Miyashiro to spy on Tsubaki. She reappears later alongside her boyfriend Marco to eliminate the threat posed by Yuki and Yuno.
When first introduced, Marco and Ai were using apprentice diaries, which Yuki and his allies mistakenly believed to be their real Future Diaries. Marco's Unbeatable Brawler Diary predicted attacks before they occurred, while Ai's Flirting Diary identified any man she wanted to flirt with in the future. Their true diaries are the Exchange Diaries, which report each other's immediate futures. The couple demonstrates remarkable teamwork, using their diaries to compensate for each other's weaknesses.
Marco and Ai are orphans who fell in love after Ai was adopted by Kamado Ueshita, Marco's guardian. Their bond is so strong that they aim to win the game and become God to live eternally together. During high school, Ai was raped by a group of girls who were "annoyed" by her behavior at school.
Marco and Ai are named after the Roman gods Mars and Venus, respectively.
