Speed-o'-Sound Sonic/Synopsis

History
Sonic was born in the ninja village and trained in its ways since birth. He was a member of the same class as Flashy Flash and the sole survivor of Flashy Flash's massacre. Despite this, Sonic held no grudge toward his former classmate.

Despite his best efforts, he was always placed in Class 5, the lowest class and the most harshly-trained class as well.

While he was training, he met Flashy Flash, who was purposefully performing badly to get the harshest training. Sonic, not knowing this, came up to Flashy Flash and told him he'd give Flash tips with bladed weapons. He introduces himself, with Flashy Flash doing the same, and warns Sonic that even talking like this could be punishable. Despite this, the two ninjas bond and train with one another due to their similar physiques and become friends.

One night, when Sonic and Flash are training in the wilderness and sitting by the fire, Sonic reveals his past and his plans to become the strongest ninja and completely change the way the village works so that there can be more freedom amongst the ninja. Sonic can tell by the look in Flashy Flash's eyes that he has the same passion.

Several years passed, and Flashy Flash had risen to be number one in rank, while Sonic had unfortunately still been unable to get out of Class 5. Before the final examination, Blast attacked the Ninja Village and seemly killed all the trainees of the 44th class with only Sonic managed to escape. One day, Sonic has a sudden case of diarrhea and was spared from death which Flashy Flash gave to everyone in the village. However, unbeknownst to Sonic, his meal was actually poisoned by Flashy Flash, possibly done as a mercy kill to Sonic for the bond they had together. Despite ingesting the poison, Sonic's durable body and strong immune system he gained from all his training in Class 5 allowed him to survive with only diarrhea.

Sometime after this incident, Sonic's previous ambitions of freedom and fulfilling his dreams were affected by his sudden introduction to the real world, and he began fulfilling odd jobs that required his assassination skills. Despite learning about Sonic's survival and the bounty he'd gained, Flash decided not to pursue Sonic. This could be interpreted as the sense of friendship Flash has towards Sonic.

Paradise Group Arc
Sonic first appears as a bodyguard contracted by Zeniru. He leaves Zeniru's tower to kill the Paradisers, who are trying to kill Zeniru. Sonic confronts the group and their leader, Hammerhead, in the woods near the tower and quickly dispatches all of them but Hammerhead, who protects his head with his suit. A brief scuffle ensues between the terrorist and the ninja until Sonic hits him in the back of his head with a kunai. Assuming Hammerhead dead, Sonic calls Zeniru to report his success. However, when he turns back, he sees that Hammerhead's body is gone. Sonic pursues Hammerhead but encounters Saitama first. He accuses Saitama of being a Paradiser. Suddenly, Sonic throws a kunai at Saitama and lunges with his sword, but Saitama counters both attacks and breaks the ninja's sword. Saitama tries to explain that he is not a Paradiser but a hero, but Sonic disregards this and says that his pride will not let him leave someone who can read his movements alive.

Sonic runs around Saitama and shows off his speed. He then jumps at Saitama, but Saitama reads his movements. Astonished, Sonic uses Wind Blade Kick to attack the hero, but his momentum carries his crotch into Saitama's fist. Sonic declares Saitama his rival as he writhes in pain from the blow.

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.

Monster Association Arc
While training, Sonic is attacked by 2 ninjas, Gale Wind and Hellfire Flame. His seniors reveal they have joined the Monster Association and they give him a Monster Cell to eat. Thinking he can beat Saitama if he gains the power of a monster, he cooks and eats the cell. Sonic gets a stomachache afterwards. He eventually spends a whole night in the bathroom to combat diarrhea and swears revenge on Gale Wind and Hellfire Flame for not telling him about the stomachache. Afterwards, Sonic feels that he has become lighter in his body weight and assumes that the monsterfication has begun but in truth he just lost two kilograms from severe diarrhea.

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.

Ninjas Arc
Sonic is seen watching the Internet Surfers wreaking havoc in City E. The Internet Surfers see Sonic, annoyed that he's just standing silently, and tell him that their "despair" isn't something they can see from afar. Sonic says that he has no business with them, and says he's waiting for a certain hero. One of the Internet Surfers thinks that Sonic plans to film them and reveal them online, while one of the other Internet Surfers explains that their antennae can intercept the signals he's sending, which is pissing them off. They prepare to kill him, but Sonic declares it's useless and that they're too slow.

At that moment, Saitama, Genos and Flashy Flash arrive at the scene. The Internet Surfers notice their arrival, but declare that no matter how many heroes there are, it won't make any difference. Sonic is pleased to see that Saitama actually came, making watching the boring battle worth it. Sonic takes off his hood, revealing his face. He notices Flashy Flash and asks Saitama why he's with him. The heroes converse with one another while the Internet Surfers, enraged that they were starting a conversation while they were in the middle. They reclarify their disaster level demon status, and state that those who underestimate them will die. Genos, Sonic and Flash all attack: Genos rips the head off one of the Internet Surfers, while Flash kills the other two with his shurikens. Flashy Flash looks back and is surprised to see Sonic had attacked Saitama but was slammed into the ground with only a single punch, marking his 14th defeat at the hands of Saitama.

After Sonic is once again defeated by Saitama and is recovering, he is approached by the Heavenly Ninja Party. This party consists of ninjas from previous graduating classes of the village, including Gale Wind and Hellfire Flame, and reveal to Sonic their plans to kill Flashy Flash and Blast, make the leader of the ninja village their leader, and come out of the shadows to take over the world with their superior ninjitsu abilities. They tell Sonic to lure Flashy Flash out, where they'll ambush and kill him. They also warn him that those who betray them will be executed. Sonic, being outnumbered, has no choice and complies to their plan.

Sonic lures out Flashy Flash, and the two begin to battle. As they fight, Flashy Flash is impressed by how much stronger Sonic has become. Sonic states that he's not the same fighter from several years ago. Flash realizes this, and concludes that he has to use his ultimate moves to defeat him. It is at this point that the other ninjas attack Flashy Flash simultaneously with their weapons. The hero is able to dodge all the weapons, but is surprised by the number of people there and asks what this is all about. The fighters reveal themselves as ninjas from the village and their plan to kill Flashy Flash. But then, Sonic attacks one of the ninjas. He goes on to say that he could handle Flashy Flash, and for them to get out of his way. The ninja reveals his Ten Shadow Burial, impressing all of the ninjas watching him. The hero then decides to silence all the other ninjas, and their battle begins.

After disposing the entire Heavenly Ninja Party, Sonic revealed that like Flash, he intentionally lagged behind to enhance his strength to harsh training at the bottom class and prepared to duel Flash to finish their fight. Gale Wind at the last moment of his life warned them about the immense power of Ninja Village Leader and his background as a monstrous assassin. He then stated that both of them would be killed in two second. As Gale Wind breathed his last breath, he revealed the Ninja Village Leader was waiting for them in Sonic's old hideout.

When both Sonic and Flash went to face the Ninja Village Leader, Sonic revealed that prior Flash's rampage, Blast attacked the Ninja Village with all trainees in the 44th classes seemed to be perished with only Sonic managed to escape. Sonic then stated the reason why he hated heroes because to him, they were nothing more than glorified mercenaries like the ninjas trained from the Ninja Village.

As Sonic and Flash approached the hideout, they were flabbergasted when they witnessed that the Ninja Village Leader had been smashed to the ground by Saitama and all his weapons were confiscated. Saitama gave Flash the Ninja Village Leader's legendary demon blades and his reforged Instakill sword as compensation and left.

Feeling let down for not be able to face the Ninja Village Leader, Sonic admited that his multiple encounters with Saitama had made him far stronger and stated that until the day he managed to defeat Saitama, Flash would be his second strongest opponent but Flash ensured that Saitama was his disciple and he would only clash sword with Sonic only if he defeated Saitama.