Originally assumed to be two separate Diary users, Marco and Ai are in fact both the Seventh Diary user. Ai appeared early in the series working undercover as member of the Onukata cult with her friend Orin Miyashiro to spy on Tsubaki. She returns later, this time with her boyfriend, Marco to eliminate the threat that is Yuki and Yuno.
When first introduced, Marco and Ai were using apprentice diaries which were assume by Yuki and his allies as their real Future diaries with Marco's The Unbeatable Brawler Diary predicting attacks before they happened, while Ai's Flirting Diary locates any man she wants to flirt in the future. Their real diaries is the Exchange Diaries which report each other's immediate future. The couple displays excellent teamwork as they use their diaries to cover each other's weaknesses.
Marco and Ai are orphans who fell in love with each other after Ai was adopted by Kamado Ueshita, Marco's guardian. Their devotion is so strong that they want to win the game so they can become God and live for eternity together. Apparently she was raped during high school due to a group of girls who were "annoyed" with the way she acted at school.
Marco and Ai are named after the Roman gods Mars and Venus, respectively.