|— Sonic's view of heroes|
Speed-o'-Sound Sonic (音速のソニック, Onsoku no Sonikku), more commonly called Sonic, is a recurring villain and occasional anti-hero. He first appeared as a bodyguard hired by Zeniru. He is a former member of the 44th Graduation Class of the Ninja Village, nicknamed "The End." Sonic is one of the few people aware of Saitama's true strength and is a self-declared eternal rival of Saitama.
Sonic is a slim young man with an androgynous appearance. His long black hair is normally tied up in a topknot, and bangs fall over his face. A noticeable feature of his is the purple marks underneath his large gray eyes. He usually wears a tight black bodysuit highlighted by metal plating on his shins, arms, shoulders, chest, and waist, as well as a long purple scarf that falls to his legs. During his fight with Genos, Genos ripped his scarf and tore off his topknot, leaving the ninja with short hair.
Despite his normally calm demeanor, Sonic loves fighting, motivated by bloodlust against weak opponents, motivated by the thrill of battle and his superiority against strong opponents he can beat, and motivated by competition against a strong opponent he can't beat.
Sonic has a large ego, and when he sets his mind to killing someone, he refuses to give up until he succeeds, unwilling to tolerate the existence of someone more capable who has directly beaten him.
Sonic has no regards for human life other than his own, and even takes pride in the fact that he has never let an opponent live.
To that end, he holds considerable disdain for the world, being quite cynical, especially towards heroes. As shown when meeting Genos for the first time, he tells him that heroes can neither win against a truly strong opponent nor can they truly protect anything.
Despite the way he presents himself, Sonic has a soft side, something seen when training to fight against Saitama. During this period, he spent a great deal of time with Frank, a hunter who had almost been killed by a bear and confides with him his inner turmoil. While he is pacing himself, Speed-o-Sound finds and befriends a little piglet as well, naming it Ino, thus showing his more noble side. When Sonic leaves to search for Saitama, he gives his words of wisdom to Frank, saying that if he is too confident in his fighting style and loses, the result will always be the same. It is unclear as to how far this side of him goes to, as he was willing to warn Genos of the Deep Sea King, yet fought him when Genos interfered with his battle with Saitama.
Due to constantly losing to Saitama, Sonic is shown willing to do anything, even trading his own humanity, to defeat him by becoming a monster through the consumption of a Monster Cell provided by Gale Wind and Hellfire Flame, although he feels that becoming a monster is equal to death, as he will be no longer human.
Sonic also respects Saitama for fighting monsters and even promised to himself to never fight Saitama while the hero is fighting monsters. He holds a very negative view of the Hero Association and some S-class heroes after observing how they treat a powerful hero like Saitama like a low-ranked and worthless hero because of his looks. Ironically, he is also rather judgmental as he assumes Tatsumaki exploits her childish cuteness and that those with benign appearances are weaker than himself, displayed in his fight against Saitama and Kamikaze.
In his youth, Sonic was a very passionate boy who strove for a better life and did not allow the torturous training of the Ninja Village to turn him into a machine. He wanted to become the highest-ranking officer of the Ninja Village to change the institution and allow the trainees to live more freely. He expressed the idea that the thing with true value is the idea to live one's life freely and hoped Flash would assist him with his dreams.
Sonic's disdain for heroes appears to stem from an incident at the Village nine years ago, when Blast arrived to eliminate the hidden sect, massacring all of the Class 5 ninjas regardless of their age. Sonic was able to escape thanks to being the strongest in the group, although the experience was rather traumatizing as it changed his perception of the world and heroes. He believes that the moniker of "hero" isn't really suited for anyone, feeling none are worthy of such a lofty title. Furthermore, Sonic feels what the hero industry does isn't at all different from what the assassination business of the Ninja Village did, with heroes merely acting as though they do it for the greater good.
Greatly loathing individuals who think highly of themselves because they are more powerful, often touting it as incentive to be self-righteous jerks. Going on to voice his general disgust and appallment at how the Hero Association and especially Neo Heroes methods make them more rotten than the criminal underworld when the latter recruited degenerates and the former tried to strike a deal with ne'er-do-wells and psychopaths as a means to an end.
Abilities and Powers
As a veteran ninja and assassin, Sonic is a master of ninja weaponry, possesses incredible amounts of speed, and is adept at hiding his presence. Fubuki commented that Sonic's fighting ability is that of an S-Class individual. However, he is rather helpless against Dragon-level monsters, as seen in his fights with Gale Wind, Hellfire Flame, and Energiser.
As a child, he was revealed to have been rather weak in comparison to other ninjas, as he was always placed in the lowest class and was unable to advance to higher classes despite being given the harshest training in Class 5. However, Sonic revealed that like Flashy Flash, he only acted like he was weak in order to receive the harshest training.
After his self-training, he appears to have found a more effective way to increase his power and was able to fight on-par with Flashy Flash, surprising the hero. What's even more astounding is that he was able to withstand one of Flashy Flash's ultimate moves: Flashy Kick. Additionally, while fighting alongside Flashy Flash, Sonic was strong enough to eliminate all 21 members of the Heavenly Ninja Party.
Immense Speed and Reflexes: Sonic's main power lies in his immense speed and reflexes. He was able to easily dodge rocks thrown by Hammerhead, as well as being able to move so fast that the latter could not even see him. Sonic was also able to easily dodge attacks from Deep Sea King, a Demon-level monster that could easily outmaneuver A-Class heroes, only had difficulty when the monster entered its true form. It has been stated by Murata that Sonic is faster than the S-Class hero Metal Bat. During his fight with Genos, an S-Class hero, Sonic was capable of keeping up with his high-speed movements while Fubuki could not even see the cyborg hero.
He could easily catch up to Tatsumaki during the fight between her and Saitama. Additionally, he appears to have become significantly stronger and faster due to his fights with Saitama, as he was able to surprise Flashy Flash several times during their sparring match.
Enhanced Durability: Sonic's durability is greater than the average human's. He withstood Base B's shoulder and dislocated the prisoner's shoulder in turn.
During his fight with Flashy Flash, his durability was shown to have significantly increased, and he was able to withstand Flashy Flash's Flashy Kick.
He also withstood Brawny Muscle's special punch, who can be considered the Heavenly Ninja Party's physically strongest member. He credited this durability to having experienced Saitama's attacks over a dozen times.
Ninjutsu Master: Sonic is a master of ninjutsu and has knowledge of various deadly techniques.
- Full Frontal Attack (正面からまっすぐに狩る, Shōmen kara massugu ni karu): Sonic takes a low stance and rushes forward at speeds that make him impossible to be seen. The effect of this attack is not known because it was interrupted by Hammerhead.
- Wind Blade Kick (風刃脚, Fūjinkyaku): Sonic flips forward multiple times extremely quickly and uses his momentum to deliver a kick to his opponent.
- Hail of Carnage (殺戮乱陣, Satsuriku Ran-jin): Sonic throws a hail of Exploding Shuriken, causing destruction to his surroundings.
- Exploding Shuriken Ferocious Barrage (爆裂手裏剣 猛襲陣, Bakuretsu Shuriken Mōshū-jin): Sonic throws a barrage of Homing Exploding Shuriken that chase down and explode upon impact with his target.
- Two Shadows Burial (二影葬, Niei-sō): Sonic creates two afterimages.
- Sonic Heavy Barrage (音速重連拳, Onsoku Jūren Ken): Sonic delivers multiple punches to his opponent.
- Four Shadows Burial (四影葬, Shiei-sō, Viz: Fourfold Funeral): Sonic creates four afterimages using a special movement technique.
- Scattered Flash Slash (散閃斬, Sansenzan): Used in a conjunction with Four Shadows Burial; Sonic attacks his opponent with his four afterimages. This was used against Genos. However, the effectiveness of the attack is unknown, as Saitama saved Genos before Sonic could hit him.
- Ten Shadows Burial (十影葬, Jūei-sō, Viz: Tenfold Funeral): Sonic creates ten afterimages. When using this technique, Sonic reaches massively hypersonic speeds.
- Serious Side Hops (マジ反復横とび, Maji Hanpukuyokotobi): Sonic can perform an inferior imitation of Saitama's Serious Series: Serious Side Hops. Sonic hops from side-to-side rapidly. Doing so creates a multitude of afterimages in the form of a wall. Sonic then charges at his opponent, however, the resulting shock wave was not strong enough to take down any members of the Heavenly Ninja Party and left Sonic exhausted.
- Speed of Sound Electric Discharge Fist (音速放雷拳, Onsoku Hōrai Ken):  Sonic extends his finger and pokes his opponents on the head, unleashing powerful shots of electricity and killing them. This move managed to easily kill all the Neo Heroes sent to eliminate him despite possessing their battle suits.
Ninja Weaponry: Sonic uses a variety of ninja-based weaponry in combat.
- Ninjatō: Sonic's primary weapon. The sword has a straight blade and a hilt and is sheathed in a black scabbard. It is extremely sharp and capable of cutting through bone.
- Kunai: Sonic carries a number of kunai and is exceptionally skilled in their usage.
- Mizugumo (水蜘蛛): Literally translated to "Water Spider", wooden sandals used to walk on water.
- Exploding Shuriken (爆裂手裏剣, Bakuretsu shuriken): Shuriken that explode upon impact.
- Homing Exploding Shuriken (ホーミング 爆裂手裏剣, Hōmingu Bakuretsu Shuriken): Shuriken that home onto a target and explode upon impact.
- Katana: Saitama stole two katanas from the Ninja Village Leader called Legendary Demon Swords (伝説妖刀, Densetsu Yōtō) and gave one each to Speed-o'-Sound Sonic and Flashy Flash.
- Smokescreen Shuriken (煙幕手裏剣, Enmaku shuriken): Shuriken that leave a trail of smoke. Sonic uses them to conceal his position.
- (To Frank) "If you are too confident in your fighting style but lose, the result will always be the same."
- (Referring to Saitama) "Whenever I face you, I cannot picture myself winning for some reason. Unless I defeat you and wipe out this thought, I cannot move on."
- Sonic is ranked 3rd in the character popularity poll.
- Sonic's prisoner number was 4188.
- ONE has said that Sonic with a weapon could have defeated the Deep Sea King pre-rain form.
- ONE stated a fight between him and Genos would end in a draw.
- As proof of this statement, when Genos and Sonic fight it ends in a draw, although technically Saitama intervened in this battle, preventing it from being settled.
- Sonic buys his weapons from a weapon shop.
- Sonic's black lines under his eyes are a reference to an American football player and are inspired by Eyeshield 21, another manga by Yusuke Murata.
- Murata revealed during a stream that Sonic and Flashy Flash are "somewhat" related. Chapters 78 and 98 later confirmed this directly, revealing that the two were from the Ninja Village's 44th graduation class, "The End." 
- In the 2017 One Punch Man Halloween cover, Sonic is depicted as a black cat, possibly a reference to the Bakeneko and therefore matching the depiction of Flashy Flash as a feline mythological creature.
- Further information in webcomic chapters 114-116 reveals that during their class graduation, Flashy Flash killed all the other members of his class, but Sonic was sick and spared from death by Flash as both of them were friends.
|S-Class||Gale Wind • Hellfire Flame • Speed-o'-Sound Sonic|
|A-Class||Bull-Bull • Serial Bomber|
|C-Class||Banero • Don Pacino • Pacino Family|
|Ex-Villains||Dr. Genus • Hammerhead • Men's Esthetician Man|
|Undetermined Class||Aburabozu • Base B • Burigura • Chimagusa • Coalition of Assassins • Cow's Stomach • Demolition John • Devil Storm • Fukegao • Harold • Paradisers • Smelly Lid Old Man • Special Moves Mile|