Speed-o'-Sound Sonic, also known as Sonic Velocidade do Som, is a recurring figure first appearing as a bodyguard for the wealthy Zeniru. Originating from the Ninja Village's 44th Graduation Class where he earned the nickname "The End," he survived its brutal Class 5 training by feigning weakness. Nine years ago, the hero Blast attacked the village, massacring Class 5 ninjas indiscriminately. Sonic escaped due to his strength but was deeply traumatized, fostering profound cynicism toward heroes and institutions like the Hero Association and Neo Heroes, which he deems corrupt. He views heroism as a facade for self-interest, akin to assassination. Sonic possesses a slim, average-height build with long black hair usually tied in a topknot, though cut short in a battle with Genos before regrowing. Distinctive purple markings sit beneath his large gray eyes. His signature attire is a tight black latex bodysuit featuring metal plating on his shins, arms, shoulders, chest, and waist, accented by a long purple scarf torn during the Genos fight but later restored. His personality blends calm intensity, combat bloodlust, and a massive ego. He refuses defeat, relentlessly pursuing those who best him, particularly declaring Saitama his eternal rival. He exhibits no regard for human life outside his own, taking pride in never sparing opponents. Despite this ruthlessness, moments reveal a softer side, like befriending a piglet named Ino during training or unexpectedly warning Genos about the Deep Sea King. He respects Saitama's strength and integrity, vowing never to interfere in Saitama's monster battles. His disdain extends to heroes he perceives as leveraging benign appearances, such as initially underestimating Tatsumaki. Fubuki assesses his abilities as matching S-Class heroes. As a ninjutsu master, his defining attribute is immense speed, enabling movement faster than perception and the creation of afterimages through techniques like Four Shadows Burial and Ten Shadows Burial. His physical capabilities include enhanced strength, dislocating shoulders with minimal effort, and durability honed by surviving Saitama's blows. Key techniques encompass the Wind Blade Kick, Exploding Shuriken Ferocious Barrage, and later developments like the Thirteen Shadows Burial and Speed of Sound Electric Discharge Fist, channeling electricity through precise strikes. His arsenal features a sharp Ninjatō, kunai, exploding shuriken, and smokescreen variants. He temporarily wielded one of the Legendary Demon Blades gifted by Saitama during the Monster Association saga. Sonic's development is fueled by repeated defeats from Saitama, driving intense training that significantly boosts his speed and combat effectiveness. This progression allows him to briefly contend with the top-tier speed-focused S-Class hero Flashy Flash, later collaborating against threats like the Heavenly Ninja Party. Desperate to surpass Saitama, he consumed a Monster Cell offered by Gale Wind and Hellfire Flame; it failed to transform him, causing severe gastrointestinal distress instead. Post-Monster Association events show him operating independently, confronting Neo Heroes operatives and navigating his complex rivalry with Saitama, whom he grudgingly admires despite conflicts. His journey reflects a continual pursuit of strength while grappling with cynicism and occasional moral ambiguities.