Speed-o'-Sound Sonic, real name Sonic, hailed from the 44th Graduation Class of the Ninja Village, earning the moniker "The End." Trained as an elite assassin, he survived a massacre by the hero Blast 15 years prior, an event cementing his cynical view of heroes and institutions, forming the bedrock of his identity as an S-Class criminal and freelance ninja who accepts missions as a bodyguard or hitman. He possesses a slender, athletic build, standing 5'9" (1.74 m) and weighing 64 kg. His long black hair is typically tied in a topknot with bangs framing his face, complemented by sharp gray eyes and distinctive purple marks beneath each eye. Sonic favors a tight black bodysuit reinforced with metal plating on the shins, forearms, shoulders, and chest, accessorized by a long purple scarf, prioritizing mobility for high-speed combat. Sonic's personality blends arrogance, fierce competitiveness, and a bloodlust for challenging opponents. He openly derides heroes as hypocrites incapable of defeating true strength or protecting anything. An obsessive drive to defeat Saitama, sparked by an initial humiliating loss, fuels relentless training. Despite his ruthless exterior, he exhibits flashes of compassion, such as befriending bear hunter Frank and caring for a piglet named Ino during training. He adheres to a personal code, refusing to fight Saitama during monster battles and expressing contempt for corrupt organizations like the Hero Association and Neo Heroes. His primary abilities center on immense speed and ninjutsu mastery: He travels at supersonic speeds (Mach 1), creating afterimages to confound opponents using techniques like Four Shadows Burial, Ten Shadows Burial, and later Thirteen Shadows Burial. He executes high-speed attacks including the Wind Blade Kick, Hail of Carnage (an exploding shuriken barrage), and the Speed of Sound Electric Discharge Fist (a lethal electric touch). He possesses enhanced strength, dislocating limbs effortlessly, and durability, withstanding explosions and S-Class physical impacts—attributes he credits to surviving Saitama's punches. His arsenal includes a ninjatō, kunai, and specialized shuriken (exploding, homing, smokescreen), later acquiring one of the Legendary Demon Swords from Saitama. Sonic's combat feats include overwhelming Demon-level threats like the Deep Sea King (though he acknowledged needing weapons for a kill), fighting Genos to a stalemate in a battle Fubuki noted was invisible to her, matching Flashy Flash's speed in brief clashes, and contributing against the Heavenly Ninja Party. He resisted the god entity's corruption via cubes during the Empty Void confrontation, demonstrating superior willpower compared to other ninjas. His trajectory shifted from recurring villain to anti-hero. Early humiliations by Saitama fueled intense training and a temporary, failed resort to monster cells (due to cooking them). Recent arcs show provisional cooperation with heroes, notably aiding Flashy Flash against Empty Void and rejecting the god's influence, reflecting a move from pure rivalry toward nuanced principles while remaining dedicated to surpassing Saitama through his own strength. Key relationships define his path: Saitama is viewed as his "eternal rival," with defeats motivating growth alongside grudging respect. Flashy Flash serves as a benchmark for his abilities, their shared Ninja Village past fueling Sonic's drive to prove himself. Genos is an intermittent combatant whose S-Class status contrasts Sonic's outsider role. Weaknesses include overconfidence, reliance on weapons for certain opponents, exhaustion from techniques like Serious Side Hops, and the vulnerability of his afterimages to dispersion under sufficient force.