Sonic is a villain, first appearing as a bodyguard hired by Zeniru. He views Saitama as his eternal rival.

Sonic is a thin, slightly effeminate man, who despite his slender build is extremely strong and fast. He has black medium length hair that is tied up in a topknot. One noticeable feature is that he has two purple marks under his eyes, one under each. He first appears wearing a tight black outfit highlighted by metal plating on the shins, fore-arms, shoulders and chest, as well as a long purple scarf. He is shown later in black street clothes. He always carries a ninjato and other ninja equipment such as kunai and ninja stars.

Sonic is a villain who, despite his normally calm appearance, loves fighting strong opponents as shown by when he gets a “child-like smile” on his face when he first meets Saitama and has two of his attacks blocked, earlier calling this his “bad habit.” He seems to be very quick to act as he attacks Saitama on their first meeting without letting him explain the situation.

After being defeated by Saitama for the first time (episode 4), while working as a bodyguard for Zeniru, he devotes his time into finding a way to defeat him.

(Source: One Punch-Man Wiki)