Takao Hiyama

Takao Hiyama (火山 高夫, Hiyama Takao) is the Third diary owner. Ostensibly the homeroom teacher of Yukiteru Amano and Yuno Gasai, Hiyama moonlights as a serial killer, hunting down women and killing them with a large knife or machete. He owns the "Murder Diary".

Personality
Little of Hiyama's personality is revealed during his short time in the story, but as a teacher he enjoys dropping surprise math tests on his students, and is shown to be a bit strict, however he could be seen as friendly at times, and this was shown in the scene where he allows Yuno to make her clay model of Muru Muru, though it only may be because of the reason that she claimed it was a brain exercise.

It seems as if his true self is a manipulative sadist, when he was shown to warn his class to be aware about the serial killer, even though the serial killer was revealed to be Hiyama himself. Hiyama is also shown to be very paranoid, as he was known to destroy his Future Diary twice when Muru Muru first introduced it to him, yet he still believed that she was recording his crime on the diary, and again under the belief it is evidence. Muru Muru also predicted that he would be the first Diary holder to die due to his paranoid nature and it was shown to be correct.

Diary & Abilities
Hiyama's Future Diary is the "Murder Diary", which predicts how and where he will kill his next victim, and how best to corner and kill them. This also works when he identifies several of the diary owners, allowing him easily track them down. However, his diary has weaknesses, namely if his victim contradicts the entries on his diary (namely fighting back) then it is rendered useless.

As the serial killer he is, Hiyama wears a hat, gas mask with googles and a bulletproof and apparently explosive proof jacket. This makes him impervious to almost anything except few things. His trademark weapon is a large machete-like knife.

Despite of this he does have many weaknesses, his paranoid demeanour along with the fact he merely corners his victims and stab-slice them with his knife. With such a simple fighting method, Hiyama is not really as smart or cunning as many of the other diary users, and his own diary only shows him how to corner his victims but not how to defend against them, which has been exploited several times across the series.

Overall, this makes him the weakest of the participants, which Muru Muru reinforces by believing he would be the first participant to be eliminated from the Survival Game.

The Survival Game
Takao Hiyama was a killer at least two years prior to the events of the Survival Game. He appears in the first episode, warning the students in his class, which include Yuno and Yuki, to be aware of the serial killer, even though he was the serial killer himself, evidenced by the fact that he the same night stalked a woman and later murdered her.

Muru Muru appeared and presented him his Future Diary, however as paranoid as Hiyama was shown to be, it did in fact take three attempts for him to realize it recorded his future and that Muru Muru was not recording the crime he committed.

The following day Hiyama went to realize that Yukiteru also was a Diary owner himself after he was taken in acing his math test using the results shown on his Future Diary. Hiyama also encountered another Diary owner at the old art university the same day, 9th, Minene Uryu, however Minene fled from him to join forces with Masumi Nishijima to escape him.

Later Hiyama is shown to target Yukiteru and follows him to a building, unaware that Yuno, the 2nd Diary owner, in fact was helping Yuki and averting his death. As Hiyama reaches the roof, he is unable to find both Yuki and Yuno, and he quickly pulls out his diary to locate his targets. At this point Yuno runs out to distract Hiyama, and as Yuki leaps up from his hiding place, he tosses one of his throwing darts and shatters Hiyama's diary. Hiyama is quickly erased from existence, much to the surprise of Yuki, and it happens at this points that the other diary owners concludes Yuki to the most dangerous diary owner, prompting most of them to target him.

The Final Battle
Hiyama is reborn towards the end of the story when Yuno, Muru Muru, Yuki and Minene jump back in time to the third world, shown to be murdering a woman. He witnesses the explosions occuring at Sakurami Elementary School, thus allowing one of his potential victims escape while he is enthralled with the sight. He then continues his rampage only to be tackled by Yomotsu Hirasaka. Two years later, he is shown to still be a criminal and on the run from the authorities. In the anime, Hiyama is captured by Yomotsu and is seen in prison; Yomotsu appears to visit him regularly, much to Hiyama's annoyance, although he does not seem to bear any sort of grudge.

Future Diary: Mosaic
Hiyama also appears in the spin-off Future Diary: Paradox, stabbing Yuki after Muru Muru accidentally destroyed the intended timeline by killing Yuno. Aru Akise saves Yuki and takes his place in the survival game, Hiyama trying to kill him but ends up stabbing a fusebox with his machete, electrocuting himself to death. However, he is resurrected by Muru Muru to restart the game, but the events of Paradox are erased, leading to his death again.

Trivia

 * Hiyama is named after Vulcan, the Roman god of fire and volcanoes. Interestingly, his surname, 火山, means "volcano".
 * Hiyama's character theme song is "THIRD/Antares Cr302" by Aki Hata.