Speed-o'-Sound Sonic (音速のソニック, Onsoku no Sonikku) is a reccurring villain and occasional anti-hero in the series, first appearing as a bodyguard hired by Zeniru and is a self-declared eternal rival of Saitama.
Sonic is a slim young man with an androgynous appearance. He has long black hair tied up in a topknot, and large gray eyes. The bangs of his hair fall down to his face. A noticeable feature of his are the purple marks underneath his eyes. He usually wears a tight black bodysuit highlighted by metal plating on his shins, arms, shoulders, chest, and waist, as well as a purple scarf which is long and falls to his legs. After the fight with Genos, Sonic loses his topknot and is left with loose shortened hair.
Despite his normally calm appearance, Sonic loves fighting strong opponents as shown by when he gets a "child-like smile" on his face when he first meets Saitama and has two of his attacks blocked, earlier calling this his "bad habit." He seems to be very quick to act as he attacks Saitama on their first meeting without letting him explain the situation.
He is persistent, even when he realizes that the opponent is much stronger than he is. When Sonic recognizes an opponent as his rival, he does whatever he can to initiate the battle, such as causing massive property damage and endangering the surrounding citizens in order to get Saitama to fight him. He also seems to have no qualms with killing as shown when he kills Hammerhead's gang and even goes as far as to take pride in the fact that he never lets a single person live, although it is currently unknown if he has killed innocent lives or has no problem with killing civilians.
He seems to be rather disdainful of heroes. When he meets Genos, 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, he seems to actually have a soft side. This side is seen when he is 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 training, he finds and befriends a little piglet as well, naming it Ino, thus showing his more noble side. When he 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 dying, since he will be no longer human. At the same time, he is also shown to have some class as he cooks the Monster Cell properly before consumption since he cannot stand raw food.
However, 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 towards 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.
Abilities and Powers
As a ninja, Sonic is adept at hiding his presence. He has displayed proficiency with multiple weapons. Fubuki commented that Sonic's fighting ability is that of a S-Class individual. It was also mentioned by Gale Wind that Sonic and Flashy Flash are the sole survivor of the 44th graduation class, "The End", from their village. However, against Dragon-level monsters, he is rather helpless, as seen with Gale Wind, Hellfire Flame, and Energiser.
Immense Speed and Reflexes: Sonic's main ability is his extreme speed. He has been shown to be capable of dodging rocks and trees thrown at him with incredible force. He is capable of running faster than what the human eye can see, appearing as a blur.
Enhanced Strength: Despite his muscular, but thin built; Sonic's power doesn't only lie in his speed. He also has superhuman levels of strength, being capable of dislocating a man's shoulder simply by walking into him.
Enhanced Durability: Although Sonic's greatest ability is his speed, his durability is much greater than the average human's, as shown when a large man walks into him to start a fight, but ends up with a dislocated shoulder.
Ninjutsu Master: As an experienced ninja, Sonic is a master of ninjutsu and has knowledge of various deadly techniques. He displayed multiple techniques involving his incredible speed and weapons.
- 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 throws himself at his foe and does a front flip, during which he throws his foot out to kick his opponent using the momentum of the flip.
- Hail of Carnage (殺戮乱陣, Satsuriku Ran-jin): Sonic leaps around throwing his exploding shuriken at cars, buildings and people to cause massive amounts of panic and mayhem. 
- Four Shadows Burial (四影葬, Shiei-sō): Sonic creates four identical afterimages through a special movement technique.
- Scattered Flash Slash (散閃斬, Sansenzan): Used in a combo with the Four Shadows Burial, the four afterimages surround the target at the same time with the ninjatō.
- Ten Shadows Burial (十影葬, Jūei-sō): Identical to four shadows burial, but with ten identical afterimages in total. During this technique, Sonic enters massively hypersonic speeds.
Ninja Weaponry: Sonic carries a variety of ninja based weaponry with him at all times.
- Ninjato: Sonic carries a sword with a straight blade, a black scabbard and hilt. It is extremely sharp, capable of cutting through bone.
- Kunai: He carries an undetermined number of kunai with which he has shown exceptional ability.
- Exploding Shuriken (爆裂手裏剣, Bakuretsu shuriken): Sonic carries with him shuriken that explode upon impact.
- Homing Exploding Shuriken (爆裂手裏剣, Bakuretsu shuriken): Sonic carries with him shuriken that explode upon impact and home onto a target.
- Smokescreen Shuriken (煙幕手裏剣, Enmaku shuriken): Sonic carries shuriken that leaves a trail of smoke. They are used to conceal his position.
- (Referring to Saitama) "Whenever I face you, I cannot picture myself winning for some reason... Unless I defeat you and wipe off 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 would 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 95 later confirmed this directly, revealing that the two were from the Ninja Village's 44th graduation class, "The End." 
|S-Class||Garou • Speed-o'-Sound Sonic|
|A-Class||Big Iron • 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|