Character from One-Punch Man by ONE & Yusuke Murata
A ninja assassin obsessed with defeating Saitama — the man who casually broke his most powerful technique and who Sonic has declared his eternal rival, a sentiment Saitama doesn't return.
Sonic is a product of the Ninja Village — the same program that produced Flashy Flash. He is a professional assassin who encountered Saitama by accident, attacked him, and discovered that Saitama was faster, stronger, and entirely unimpressed. This broke something in Sonic. He has since dedicated his existence to surpassing Saitama, which is impossible, but the pursuit gives him purpose. He is the series' most persistent recurring opponent who is never actually a threat. Saitama doesn't remember his name half the time.
Lean and androgynous with long dark hair in a ponytail and a ninja outfit. He carries a short sword and shuriken. He moves at supersonic speed — his name is literal. His expression is usually smug, which Saitama finds annoying.
Also known as: Speed-o'-Sound Sonic, Sonic