His voice should be mid range-deepish, and it should sound authoritarian.

Kiyotaka Ishimaru is a tall school hall monitor who sticks to the rules very closely due to his grandfather having a great scandal while being Prime Minister. He doesn't believe in his genius bringing him good grades, but rather the amount of time and work he puts into his school life. Because of his intensive studying, Kiyotaka lacks most social skills, and has troubles with relationships, platonic or otherwise. Despite this, he is still a caring, kind person, albeit awkward and unaware of social cues.

During the game, he still shows the same intelligent but clueless personality, with examples being how he had thought that the killer in the first trial would easily raise his hand. Later, he and Mondo, the Ultimate Gang Leader, get in a fight, which leads them to a stamina competition in the sauna. They both determinedly withstand the heat until late hours in the night, long after Makoto, who was the judge of the competition, had left. In the next morning, their relationship has done a full 180 and they're totally "bros" now. Many members of the fandom take this to be that they're in an unspoken romantic relationship. The creators of the game have even stated that it is possible they would've unknowingly fallen in love if the killing game hadn't occurred. That is besides the point.  

Ishimaru's new "bro", Mondo, goes on to murder close friend Chihiro. This, of course, gets Mondo executed in the 2nd trial. Heartbroken over losing his bro, Ishimaru falls into a deep depressive episode. It is stated that if Hagakure hadn't helped him, he would've wasted away without water, sleep, or food. Ishimaru couldn't even talk. Despite this, when it is later revealed that Chihiro had made a computer AI to find information on the killing game, Ishimaru has the computer simulate Mondo's appearance, voice, and dialogue. This renews Ishimaru's hope, and he takes on the name of Kiyondo, to live on as a personified fusion of the two men.

Tragically, this is still a killing game, and Ishimaru sadly is murdered by Hifumi, who is later murdered by Celestia. He was given a note and told to go to a storage room alone in the middle of the night. The game leaves much up to the imagination, but it has been speculated that Ishimaru could have fully known it was a trap, and went there to die. He is, after all, a genius, even though he denies it. The fusion could have been a coping mechanism, which could never truly get rid of his depression over losing his one and only "friend" to the killing game he was trapped in. Either way, he finally got his rest. 

(I asked my friend to write this because I knew they liked Ishimaru... I didn't expect it to be this long though...)