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 does allow Yuno to continue making a clay model of Muru Muru when she claims it is a brain exercise. However, it seems his true self is a manipulative sadist, warning his class to be aware of the serial killer, when he is the killer himself.

Hiyama is a paranoid man, destroying his Future Diary twice when Muru Muru first presents it to him, once when he believes she is recording his crime on the diary, and again under the belief it is evidence. Muru Muru predicted he would be the first owner to die due to his paranoid nature.

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, teaching Yuki, Yuno and his other students in class and warns them of the serial killer. That night on April 22nd, Hiyama stalked and murdered a woman. Muru Muru appeared and gave him his Future Diary although it took three attempts for Hiyama to realise it could predict the future. The following day, Hiyama realised Yuki was a diary owner after he aced his maths test using the results on his diary. Hiyama also encountered the Ninth diary owner Minene Uryu at the old art university. Minene fled from Hiyama and joined forces with Masumi Nishijima to escape him.

On April 24th, Hiyama targeted Yuki and followed him to a building, unaware that Yuno, actually the Second owner, had used her diary to reach Yuki first and avert his death. Travelling to the roof, Hiyama was unable to find Yuki and Yuno, pulling out his diary to locate his targets. It was at this point that Yuno ran out to distract Hiyama, Yuki leaping up from his hiding place and tosses a throwing dart, shattering Hiyama's diary. The shocked Hiyama is quickly erased from existence, giving Yuki a shocked look before he vanishes. The other diary owners concluded Yuki was 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, 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. His surname 火山 means 'volcano'.