Ouji Kosaka (高坂 王子, Kōsaka Ōji) is a classmate of Yukiteru Amano who bullied and teased him in the past, but later becomes his friend during the Survival Game. He owns the apprentice diary, Know as the "Kosaka King Diary".
Kosaka is a member of Sakurami Elementary School's track team and is shown to be an athletic boy. He likes to tease Yukiteru Amano, but does not outright bully him, due to Yuki's detached and easily submissive attitude. Kosaka is shown to be very self-centered, cocky, arrogant and obnoxious, willing to sell Yuki out to Minene Uryu during her attack on the school. Kosaka has a strong sense of pride and often praises his own efforts, and has quite the ego, even going as far as naming his diary as the "King Kosaka Diary", which becomes his Apprentice Diary. Upon meeting Hinata Hino, Mao Nonosaka, and Aru Akise, as well as Yuki again, Kosaka shows a more considerate and helpful side. He continues to be very self-centered although he realises his faults during his confrontation with Yuno Gasai whilst trying to save Yuki and Hinata, showing more concern for his friends and updates his diary to the "Neo King Kosaka Diary", now complimenting his own achievements and those of his friends. Kosaka is cautious and usually the most open and objective to the survival game.
Kosaka gets on best with Akise, often accompanying him on missions due to his athletic skills. He gets on quite well with Hinata and Mao, often bantering with them, although his objections and sarcasm often get remarks from Hinata and Mao. Mao often physically hits Kosaka, punching him to the ground in Double Holder. Kosaka relationship with Yuki is strained at best, going from good to bad. Kosaka often calls Yuki out, particularly on his relationship with Yuno and ultimately describes him as pathetic and has no right to cry after some of the more horrific actions he commits.
Attack of the Ninth
Kosaka first appears in the first episode, where he tries to start a conversation with Yukiteru in class, only for Yuki to rebuff him.
When the terrorist Minene Uryu attacks Sakurami Middle School, she ordered the students and the faculty to hand over Yukiteru, or else she would detonate the bombs planted throughout the school grounds. A desperate Yuki seeks help from Kosaka, who betrays him and hands him and his Future Diary over to Minene.
The Tenth's Trap
After Minene's attack, Kosaka and the rest of the students are relocated to Umesato Middle School. Kosaka verbally abuses a cowardly Yukiteru loudly to allow the whole class to hear about Yuki's stupidity. However, Hinata Hino and Mao Nonosaka come to Yuki's defence, Hinata whacking Kosaka on the head with a booklet. Kosaka later joins Hinata, Mao, Yuki and a reluctant Yuno when they explore a crime scene and play together in the park. When Hinata goes ahead and is supposedly killed by a group of vicious dogs that were responsible for the recent serial killings across Sakurami City, Aru Akise appears on the scene and guides Kosaka and the others to safety inside a nearby observation deck.
Inside, Akise explains that he had been investigating Yukiteru after he realized all the recent incidents (the fights with the other Diary owners) were somehow all related to Yuki. However, the group of dogs surround the building and begin pounding the windows to get in. While Yuno suggests escaping with Yuki while using the others as bait for the dogs, Yuki defends his new-found friends by revealing himself as a Diary owner who can read the future. Kosaka is immediately skeptical and initially refuses to follow Yukiteru's instructions regarding which window to block to stop the dogs from entering, but he eventually helps Mao and Akise as the instructions appear to succeed in hindering the advancing dogs. At this moment, Kosaka and the others congratulate Yuki before Mao withdraws a knife and presses it to Yuki's throat. Kosaka watches in disbelief as an unharmed Hinata reappears through the door, revealing herself to be the owner of her father's Breeder Diary and faking her death to secure an opportunity to kill Akise, who had been targeting her father for the recent serial killings.
Kosaka watches from the sidelines as Akise engages Hinata in a coin-toss guessing game where a player must guess the hand in which a tossed coin lands. Despite Hinata having a definite advantage by holding Yukiteru's Indifference Diary, which told her the supposed correct answer for every attempt, Akise outwits Hinata on multiple occasions to win the game, securing their release. However, Yuno attacks Mao after she threatened to stab Yuki's diary, leading to Mao getting severely injured. Akise, Kosaka, Yuki and Yuno flee, while a distressed Hinata calls all the dogs to give chase, blaming them for harming her friend. Akise and Kosaka lure the dogs away while Yuki returns to the observation deck with Yuno in tow, seeking to make peace. Nonetheless, Yuno holds Hinata at knife-point and orders her to call off the dogs, at the same time threatening her life for taking Yuki from her. Kosaka and Akise return unharmed just as Yukiteru boldly declares Yuno to be his girlfriend, which he did reluctantly because he still wished to be friends with Hinata and Mao.
- Kosaka's character theme song is "Me! Now? The Greatest" by Aki Hata and performed by Kosaka's voice actor Minoru Shiraishi.
- Kosaka's first name, Ouji, means "Prince". This serves as a joke in Future Diary Redial, where Orin Miyashiro and Ai Mikami assume Tsubaki Kasugano is in love with Kosaka ("Ouji-sama"), when she is actually referring to Akise as a prince.