Yukiteru Amano, or Yuki for short, is kid who really doesn't get along with others so he doesn't have any friends except one, Deus ex Machina. Yuki thought Deus was nothing but a imaginary friend, but that changes when Deus gives Yuki a Future Diary. Yuki's cellphone, which is what is used as daily diary, can now write future as if his future self is writing it. However he was not the only one with Future Diary and it came with a price. Turns out Deus is holding some sort of Battle Royal fight between all twelve Future Diary users, and whoever is the last one to survive becomes the next Deus ex Machina, the god himself.
- Deus Ex Machina - The God of Time and Space who is getting weaker and weaker. He has created Future Diaries and Diary Game to find his successor, the next one who will become the God of Time and Space.
- Muru Muru - Deus Ex Machina's servant who goes on and off as she please. In the beginning, she seems to be nothing but a harmless character, but as story moves on, she starts to scheme something behind Deus Ex Machina's back which can changes the game forever.
- Yukiteru Amano - Protagonist of the series, the First Diary user. Before the event of Diary Game, Yuki was just a kid who would not socially interact with other kids. Only things he did for fun was to write diary on his cellphone and talk with Deus Ex Machina, whom he thought was nothing but his imaginary friend. Yuki was shocked to find out that Deus Ex Machina was indeed real and forced to participate in a battle royal called Diary Game. His Diary, Random Diary, allows him to read any future entries by himself which gives him both advantage and disadvantage at same time. His diary is written in a way that Yuki would write so his diary entries are basically everything, however at same time the entries are limited to his own point of view, meaning it will not show anything that Yuki would not write.
- Yuno Gasai - The female protagonist of the series, she is the user of Second Diary, Yukiteru Diary. Just like its name would suggest, Yuno's diary shows what Yuki does every ten minutes, since Yuno is obsessed with Yuki and stalked him even before the event of Diary Game. She is so obsessed Yuki that she will do anything for Yuki, going far as killing anybody and sacrificing her own life for him, a very Yandere-like nature.\
- Minene Uryu - Holder of Ninth Diary, Escape Diary. Minene is a terrorist whose been targeting major religious figures due to her hatred toward God, thinking that God is blame for her parent's death. She debuts as antagonist, another Diary Game contestant competing against Yuki but starts to support him later in the series. Her diary, Escape Diary shows her the best way to escape from dangerous situation.
- Takao Hiyama - The Third Diary user, who happened to be a teacher at Yuki's school. Turns out he is a serial killer who has been using diary Murder Diary to kill his victims. The Murder Diary shows him his next victims, however it does not show what kind of actions his targets will take. He was first to be eliminated by Yuki when his Diary was destroyed by Yuki's dart.
- Keigo Kurusu - Captain of Police department who is also holder of Fourth Diary. Initially he hleps Yuki and Yuno in the beginning, hoping to end any more of unnecessary death. However his mind changes when his son was about to die from his weak heart condition, which gave him desire to win the game and become a god to save his son. He used both his position as police captain and his Diary, Criminal Investigation Diary, as advantage to capture Yuki and Yuno. His Criminal Investigation Diary shows him any crimes and that will happen, however the crimes that shows on his diary is limited to crimes Keigo investigates, just like Yuki's diary.
- Tsubaki Kasugano - Holder of Sixth Diary and also a leader of cult. She did not become the leader of cult by choice;when her parents, who were the original leader of cult, were killed in a car accident she was forced to become the next leader. To make it worse for her, she was raped by cult member as part of 'ritual' which made her traumatized. Ever since the incident she hated the world since, and when she becomes a god she wishes to destroy it. Her Clairvoyance DIary, which is actually a scroll, allows her to see what her 'followers' see.
- Yomotsu Hirasaka - Holder of Twelfth Diary, Justice Diary. A blind man who is fixated on his own justice, and a fan of Japanese super heroes such as Kamen Rider or Super Sentai. Since he is blind, his Diary is in form of tape recorder, which will tell him good deed ranging from simple trash picking to capturing criminals. He wears spandex all over his body and wears round bag with one eye, and his real face was never shown.
- Reisuke Houjou - A four-year-old boy who is the holder of Fifth Diary. Despite his age, he is quite intelligent and cruel at the same time. His Diary, Hyper Vision Diary, is in form coloring book that only shows three entries per day. Compared to other Diary holders, his entries are limited but he uses that as advances to see the day's entries carefully planning his moves. It is revealed later that his parents were part of Tsubaki's cult and blames Yuki and Yuno for their death.
- Karyuudo Tsukishima - Holder of Tenth Diary, Breeder's Diary. A rich man who devotes his too much on his dogs, so much that he has neglected his duty as husband and father. His Breeder's Diary shows him activity related to his dogs, but it also allows him to control them as well.
- Marco Ikusaba - Part holder of Seventh Diary, Exchange Diary. At first Marco called his Diary The Unbeatable Brawler Diary, saying that it would show any attacks coming at him before it happens. However turns out that his Diary was actually part of Exchange Diary, which shows Marco and Ai's future entries.
- Ai Mikami - Another holder of Seventh Diary, Exchange Diary. Just like Marco she told everybody what her Diary is with different name, Flirting Diary which shows any men she wants to flirt.
- Kamado Ueshita - Holder of Eighth Diary, Blog Diary. A large women who is in charge of a orphanage, where Marco and Ai also grew up from. Her Blog Diary doesn't show future entries to her, but to people who she allowed to be "Apprentice" in her blog server, naming them "Apprentice Diary". With it just about anybody can have their own Future Diary with different way of reading future. She doesn't really participate in the fight, since all she care about is to take care of her orphanage.
- John Balks - Holder of Eleventh Diary, Watcher Dairy. A mayor of city where events of Diary Game is happening, and a man who came up with ideas of Future Diaries and Diary Game. As a man who helped Deus to make Future Diaries, his Watcher Diary allows hims to read entries of other diary users which enables to predicting other people's action.
- Aku Akise - A boy genius who wants to become a great detective. He somehow ends up getting involved of Diary Game, which pique his interest to learn the truth behind everything. As his investigation went on he discovers truths more than he bargained for.
- Masumi Nishijima - Keigo's subordinate in police department. Falls in love with Minene even after finding about her true identity.
- Hinata Hino - Classmate of Yuki. She is actually daughter of Tenth Diary user, Karyuudo Tsukishima. She was tricked by her father to kill Yuki and Yuno, saying that they can be happy family again if she does what she was told to, but turned out that he was lying from beginning.
- Rea Amano - Yuki's mother who is not at home most of times due to her job as a video game programmer.