Sonic is a slim young man with a feminine face, long black hair tied up in a topknot, and large grey eyes. A noticeable feature of his are the purple marks underneath his eyes. He usually appears wearing a tight black bodysuit highlighted by metal plating on his shins, arms, shoulders, chest, and waist, as well as a long purple scarf. 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 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. However, unlike Garou, he does not go out of his way to attack heroes.
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 Matagi, 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 him Ino, thus showing his more noble side. When he leaves to search for Saitama, he gives his words of wisdom to Matagi, 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.
Abilities and Powers
Physical AbilitiesImmense 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. His Ninjutsu training is the most probable explanation for his immense speed, but so far the only person shown to be able to surpass his speed is Saitama, although it can be assumed that warriors like Boros and Garou who have said by Saitama to be some of his most powerful enemies are faster than he is.
Enhanced Strength: Despite his thin appearance, 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: In addition to his speed and strength, he is capable of taking large amounts of damage as shown when Sonic fought the Sea King.
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: 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, Fujin Kyaku): 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: Sonic leaps around throwing his exploding shuriken at cars, buildings and people to cause massive amounts of panic and mayhem. 
- Four Shadows Burial: Sonic creates four identical afterimages through a special movement technique.
- Scattered Flash Slash: 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: Same with four shadows burial, but with ten identical afterimages in total.
EquipmentNinja 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: Sonic carries with him shuriken that explode upon impact.
- Smokescreen 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.
- ONE has said that Sonic with a weapon would have defeated the 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 has confirmed that Sonic and Flash are "somewhat" related.
- There are several similarities and contrasting differences between Flashy Flash and Speed-O'-Sound Sonic:
- Both have slim figures with feminine facial features.
- Both are 25 years old.
- Both have names ascertaining to speeds of certain inanimate forces.
- Both have a very skilled knowledge of ninjitsu.
- Both know the Wind Blade Kick and have used it in battle.
- Both are rather confident in their abilities and are extremely fast, much faster than most S class heroes.
- While Flashy Flash usually has a composed, stern, and serious demeanor, Sonic often has a flamboyant and egotistical nature.
- Flashy Flash's entire manner of dress is white, while Sonic's manner of dress is black.
- Flashy Flash is a member of the Hero Association and believes in helping people & defeating monsters, while Sonic believes that heroes are worthless and can never really save anyone or anything.
- Flashy Flash appears to solely use his katana, while Sonic has used a wide array of different ninja weapons.
|Oldface • Beefcake • Hammerhead (Paradise Group) • Banero • Bull-Bull • Speed-o'-Sound Sonic • Dr. Genus • Men's Esthetician Man • Garou • Don Pacino • Choze • God • Hamukichi • Volten • Benpatsu • Rosie • Big Iron|