Speed-o'-Sound Sonic/Synopsis

Paradise Group Arc
Sonic first appears as a bodyguard for a rich man named Zeniru, defending him from a group calling themselves the Paradise Group. Sonic appears before the group and their leader, Hammerhead, as they are approaching Zeniru's building. After appearing before the group and claiming that he has never let a single person live and quickly dispatches all of the group, except the leader who is able to block Sonic's attack in time. After avoiding and subsequently mocking Hammerheads attacks, Sonic hits him in the back of the head with a kunai. Assuming that Hammerhead has been defeated, he calls in to report his success, but when he turns back he sees that Hammerhead is gone.

Following after him, he meets Saitama and accuses him of being part of the Paradise Group. He tried to slash Saitama, but the latter managed to catch and break it. Saitama tries to explain that he isn't part of The Paradise Group and that he's actually a guy who's a hero for fun, but Sonic doesn't listen and says that his pride won't let him have had someone block him that many times. Saitama then calls him out on the fact that Sonic really just wants to fight him because he is strong.

A fight ensues that shows off Sonic's incredible speed, only to end abruptly when the ninja runs his crotch into Saitama's fist. Sonic momentarily recovers, claiming that because he has found a strong opponent he was leaving to train, adding that the next time they meet would be the last and that he would kill Saitama. All while writhing in pain from injuring his groin.

Rumored Monster Arc
Saitama is running about, worrying about losing his heroes license when Sonic throws a kunai at him and he catches it. Sonic begins to threaten Saitama, but Saitama interrupts him, saying that they could do this another time and begins to walk away. This angers Sonic who then draws his sword and rushes at Saitama to attack, but Saitama quickly turns around and bites Sonic's sword in half. Sonic stands still for a second, confused by what had just happened until Saitama threatens to punch him, causing Sonic to shudder from the menacing aura. At that point the two of them are distracted by Tanktop Tiger who has shown up to investigate the person who'd been running around town and starts to threaten Saitama, until Sonic throws several exploding shuriken at Tanktop Tiger which causes him to pass out. Observing that Saitama is a hero, Sonic declares that he will create a situation that will force Saitama to fight him, then jumps back and throws several more exploding shuriken which Saitama is prepared to block, but instead fly off in different directions, hitting the surrounding buildings and causing a lot of mayhem. Sonic continues to do this until Saitama jumps up behind him and chops him in the back of the head which throws him to the ground hard enough that it leaves an impression of his gleeful face in the pavement.

Sea Monster Arc
Sonic escapes from prison when Puri-Puri-Prisoner broke out to save J-City from the Deep Sea King Sonic watches the fight between Puri-Puri Prisoner and the Deep Sea King and notices that Puri-Puri Prisoner was acting tough and is struggling and thought that he can take out both of them with ease. After the Deep Sea King defeats Puri-Puri Prisoner, he annoyed Sonic which made him want to fight the monster. Deep Sea King attempts to punch Sonic who easily dodges, causing him to attack Sonic several more times, with the ninja dodging the attacks and eventually landing a kick on Deep Sea King's face. After landing the kick a snake-like creature appears in front of Sonic and attempts to bite him, but only manages to rip his shirt. The creature is actually a long Moray that is coming out of Deep Sea King's mouth.

Deep Sea King goes to attack with his Moray again, but Sonic dodges it and smashes Deep Sea King's mouth shut, causing him to bite off the Moray. Deep Sea King then attempts to use a chain of punches on Sonic, who easily dodges and jumps at Deep Sea King, attacks him and lands on the roof of a nearby building. However when he looks back, the Deep Sea King is laughing. Deep Sea King the launches himself at Sonic and crashes into the roof of the building, with Sonic barely being able to dodge in time. After dodging away from Deep Sea King's attack, Sonic jumps off to a different building, but before he has a chance to do anything, Deep Sea King shows up. He then reveals that after he'd left the sea his body had shriveled up, but the rain enabled him to return to his true form. Sonic manages to escape, leaving behind his prison outfit in the Deep Sea King's grip. Sonic then appears on a nearby roof naked, vowing that the next time the two would meet would be their last, after which he leaves. Then he meets Genos on the way who asked him who he was. Instead, he warned him about the Deep Sea King and leaves. Sonic was later spotted by Saitama, who at the time was tagging along with Mumen Rider. Sonic returns later on with his full ninja outfit, prepared to fight the Deep Sea King once more, but is too late, as Saitama had already eliminated the monster.

Alien Conquerors Arc
Sonic was listening to the radio in his hideout, about the destruction of A-City, as he was polishing his sword. After the alien invasion was over, leaving the trace of their technology parts from the ship, Sonic investigate the metal wall that was punched by Saitama to get out of the wrecked ship.

Garou Introduction Arc
Sonic and other criminals was invited by Sitch in an attempt to assist the Hero Association. Sonic managed to steal the paper, before they were passed around. Sonic explains the meaning of the gathering and declines the offer, stating he is only interested about Saitama. As he was leaving the Headquarters, he heard commotion inside caused by Garou, but brushes it off, stating he wasted his time for not finding Saitama, declaring he will defeat him.

The Blizzard Group Arc
His persistence to beat Saitama continues as he went to his place in order to challenge the hero once more. However, he ended up facing Genos who introduced himself as Saitama's disciple. They begin their battle and are shown to be on equal footing. Genos couldn't catch up with Sonic's speed, but his attacks weren't hurting Genos either. Genos attacks him again but Sonic evaded. The impact caught Fubuki, who was fighting Saitama at that time, but he quickly got between Fubuki and the explosion. - Sonic retaliates with exploding shurikens, but is still unable to hurt Genos, who manages to cut off Sonic's topknot. Saitama stops the two of them from fighting each other and then accepts Sonic's invitation to a fight. Sonic reveals his newly learned technique, but still questioning whether he can beat Saitama. Saitama then uses one of his "Serious Series" moves to beat Sonic, who once again promises that he will defeat Saitama the next time they meet and then vanishes.

The following section contains Webcomic spoilers. You have been warned manga-only readers.

Psychic Sisters Arc
While Saitama fights Tatsumaki, Sonic notices the two and tries to attack Saitama with Homing Shuriken. However, Tatsumaki sends them flying back at him. Sonic dodges most of the shurikens, but slips on dog feces and the last few hit him and explode.