Ouji Kousaka

Another one of Yukiteru's classmate, Ouji is in Yukki and Yuno's classroom during the previous semester and is one of the first people to betray him during the terrorist bombing incident. He writes down a list of all of his proudest accomplishments in a blog he dubs the "Kousaka King Diary", often using the phrase "I'm Shining!" after each sentence.

[Spoiler] Due to having blogged many times on his cellphone, which is connected to the Eighth's diary, he becomes a "Diary Holder Apprentice". His diary helps him save his friends, but it only reveals his successes, not his failures, which makes Yuno able to shoot him with a crossbow. Since he is inadvertently working as the Eighth's spy, Yukki destroys his diary in chapter twenty-seven, though does not die because it was not a true future diary. [/Spoiler]

(Source: Wikipedia)