Sonic is a mercenary ninja
Sonic's origins are not known in any depth.
Sonic was created by a mangaka who uses the pseudonym "ONE" for his webcomic One-Punch Man in 2009. In 2012, ONE was approached by Yusuke Murata to consider a collaboration between the two, and the One-Punch Man remake was the result. Sonic was an early antagonist in the series but has taken on more of a protagonist role in later chapters.
Little of note has changed with Sonic thus far in the story.
Sonic has a very slim and athletic build that is scarred from unknown past experiences, yet he has rather soft features, giving him a somewhat androgynous element to his appearance. He usually wears a tight fitting black full body suit. When incarcerated he wore a standard white and black striped inmate uniform.
Sonic is rather unpleasant and vulgar. He seemed like a villain initially but he has some degree of a moral compass. He tried to defeat the powerful Sea King in A-City although upon realizing he was outmatched he did flee.
Sonic is introduced into the story as a hired bodyguard for Mr. Monier. Mr. Monier was targeted by a group known as the Paradise Group. The Paradise Group made their way through a forest and arrived at Mr. Monier's building but were met by Sonic. After a quick threat, Sonic disposed of the entire group aside from Hammerhead the leader who managed to defend against Sonic's first attack. However he was quickly defeated as well. Sonic called Mr. Monier to inform him of his success, but turned and found Hammerhead gone. He decided to track Hammerhead, but on the way he happened to bump into Saitama. Thinking that he was a part of the Paradise Group, Sonic attacked him with a kunai, but it was caught. He attacked with his sword but it was bitten to pieces by Saitama.
Saitama tried to explain that he was not affiliated with the Paradise Group but the stubborn Sonic would not listen and was driven to beat Saitama for no other reason than that he was somehow defending against all his attacks. Sonic resorted to his strongest aspect, that being his speed. He began blitzing through the forest, showing off his incredible speed. However went he dashed towards Saitama he ran directly into Saitama's fist with his groin. Clearly beaten, Sonic fled with haste saying that he would train now that he met a strong opponent and that he would kill Saitama the very next time they met.
Sonic arrives and A-City with Puri Puri Prisoner. The duo had just escaped prison together and decided to face off against the Sea King, who had already easily dispatched powerful Heroes prior to their arrival. Puri Puri Prisoner went first, but despite a valiant effort, was beaten with relative ease. Sonic stepped in next. He managed to avoid the Sea King's assaults entirely, even causing him to bite off his own tongue. But when the Sea King grew angry and the fact that it had begun to rain and thus, hydrated the Sea King and empowered him further, he nearly succeeded in crushing Sonic in his massive fist. After such a close call, Sonic decided that he would flee and save himself against the mighty foe, which was subsequently defeated by none other than Saitama, the same man who had recently beaten Sonic himself.
Super Speed: Sonic's primary ability lies in his immense speed. He can blitz around as if he were a blur or seemingly disappear in a burst of speed. He managed to outpace the likes of the Sea King until the latter became hydrated and nearly matched him. Even the likes of Genos could not avoid all the Sea King's blows while Sonic succeeded in doing so. Only Saitama and perhaps Lord Boros have proven themselves faster so far.
Sonic carries kunai on his person. Essentially they are daggers designed for throwing.
Basically a shorter katana but not as short as a wakizashi. They are the stereotypical blade used by shinobi as katana are for Samurai.