His voice should be mid-range to deep and have a gruff edge about it.

Mondo Owada is the tough leader of the biker gang known as "The Crazy Diamonds." This earns him the title of the Ultimate Gang leader. His brother, Daiya was the first leader of his biker gang, and Mondo admired his brother with every fiber of his being. Daiya is Mondo's role model, and he raises Mondo to inherit the gang. On the night of his brother's retirement from leading the gang, Mondo races his brother on their motorcycles to prove he's ready to take on leading the gang. Tragically, when he doesn't notice an incoming truck, Mondo is almost killed, only saved by his brother, who pushes Mondo out of the way and takes the hit instead. As his dying wish, Daiya makes Mondo promise to keep the gang together. He promises, and Mondo continues with his gang until receiving an invitation to Hope's Peak, the cite of the killing game.

In the killing game, Mondo is a gruff side character for the first chapter. In the second, however, he fights with fellow classmate Kiyotaka before being best bros after a stamina contest in the sauna. For more details, see Ishimaru Kiyotaka's description. From here on out, Mondo is more openly nice, especially with his new "bro." 

Then the second motive comes out, and Monokuma states that if a killing doesn't occur, everyone's deepest secrets will be revealed. Mondo's secret is that he killed his own brother from the aforementioned race in the first paragraph. Not wanting his gang (that he assumes to be still in tact outside of the school) to fall apart over this, he panics. When Chihiro comes to him asking for help with his secret, Mondo discovers that Chihiro is actually a boy who hides his gender to avoid bullying. To prevent humiliation from his secret being revealed, Chihiro attempts to become stronger with guidance from Mondo. Annoyed that the small boy could be trying his best while he was hopelessly trying to avoid his secret being revealed, Mondo murders Chihiro in an act of blind rage. After this, he attempts to hide Chihiro's gender to keep his promise about helping the crossdresser. 

This inevitably makes Mondo the next blackened to be executed in the second trial. His bro, Kiyotaka, tries to vote for himself because of denial and their strong bonds, but Mondo is spun in a circular cage on a motorcycle until he turns into butter.

(I asked my friend to write this because I knew she liked Mondo... I expected this one...)