Speed-o’-Sound Sonic, or simply Sonic, is a recurring antagonist from the manga/anime series One Punch Man. He is a master of ninjutsu who can move at the speed of sound. Ever since his first encounter with Saitama, he has grown a huge rivalry for him, and has now become hellbent on defeating him.
He is voiced by Yūki Kaji in Japanese and Erik Scott Kimerer (who also voiced Secco) in English. Sonic is a fighter who is easily consumed into battle. He gains pleasure from not only being involved in fights, but winning the fights at that.
Though he accepts that there are heroes and villains that are stronger than him, he refuses to be lower than him. He is a very determined man who will go out of his way to surpass others, including his rivals.
Despite his cold presence, he does have a side of compassion for others. Sometime after his first encounter with Saitama, he spent his time with a bear hunter named Frank, who grew a nice bound with him. Around this time, he also befriended a piglet which he later named Ino.