Goto Imasuji/Synopsis

History
Muscular is a vicious criminal who likes to destroy and harm others. Two years prior to the Vanguard Action Squad Invasion, Muscular attacked a village and tried to kill civilians. However, a team of heroes named Water Hose fought to protect them and sacrificed themselves. Muscular killed them both, but at the cost of his left eye.

At some point in the past, he received a medical examination from Kyudai Garaki, likely indicating that this is when Kyudai managed to copy the Muscle Augmentation Quirk.

Forest Training Camp Arc
Muscular joins the League of Villains following the Hero Killer's defeat. He's recruited by Giran to the Vanguard Action Squad and given a mask and cloak to wear. While the team waits in a warehouse to be dispatched on their first mission, Muscular mentions that they need to hurry because he's ready to fight. Most of the Vanguard Action Squad arrives at the Wild, Wild Pussycats' Beast's Forest reserve and observes the U.A. High School students training there. Muscular says he can't wait to be let loose, but Dabi tells him to relax. Once the mission to capture Katsuki Bakugo commences, Muscular searches for a high vantage point and happens upon Kota Izumi.

Muscular compliments Kota on his hat and asks to trade for the mask the League of Villains gave him. He notes that he was looking for a nice view and is surprised to find someone outside of the list Tomura Shigaraki gave the team. The battle-hungry villain removes his mask, and his young target takes the chance to run away.

Using his Quirk to boost the muscles in his legs, Muscular ricochets off the cliff side to get in front of Kota, noting with a bloodthirsty smile that he is going to have fun with the boy. He augments the muscles in his arm and removes his cloak in the process, revealing his prosthetic left eye and a large scar. Muscular strikes at his terrified victim with his quirk-enhanced arm, only to miss due to the timely intervention of Izuku Midoriya. Muscular recognizes Izuku from the aforementioned list and grows excited with anticipation.

Muscular mocks Izuku's claim to save Kota, noting that is exactly what a budding hero would say and that all heroes are always talking about justice. Muscular confirms Izuku's identity and says the villains have been cleared to kill him, but not before tormenting him first. Muscular launches himself at Izuku, punching him into the cliff wall. Before injuring Izuku too greatly, Muscular inquires about Katsuki's location because the team still has a mission to complete. He takes Izuku's silence as ignorance and attacks again. Izuku evades, but Muscular quickly follows up with a brutal kick that sends Izuku flying into the cliff wall again.

As Izuku lies bleeding on the ground, Muscular gets excited at the sight of blood and mocks the student's claim to save Kota. Izuku counterattacks and punches with his Quirk activated, only for it to be easily blocked by Muscular's quirk-enhanced arm. Muscular compliments Izuku's speed, but notes his strength isn't enough to hurt him. He explains that his quirk increases his speed and strength so exponentially that his muscle fibers can't be contained by his skin. Muscular calls himself superior to Izuku and strikes him down before mocking his desires to become a hero.

Just as he prepares to finish Izuku off, Muscular is distracted by Kota, who throws a rock at the villain. Kota asks the villain if he tortured his parents like he is doing with Izuku before he killed them. Surprised, Muscular states this must be fate, revealing that Kota's parents were the ones who gave him his prosthetic eye. Kota replies that things like this always happen when crazy people are involved. Muscular says kids are always trying to subvert blame and explains that Water Hose's deaths were the results of both parties doing what they wanted. He believes the real tragedy was Water Hose failing to do what they said they would do, and says that's why they died. Muscular prepares to kill Kota, but Izuku attacks from behind.

Muscular says he anticipated Izuku's attack and turns to face him. His arm is poised to counterattack, but Izuku lodges his own arm into the exposed muscle fibers. Izuku tells Muscular he can't evade now, but Muscular replies that he's unafraid of Izuku's puny quirk. Muscular moves to attack and Izuku charges his quirk to 100%. Muscular recognizes that Izuku's impending attack is stronger than the others. Izuku punches Muscular with a maxed out smash and believes he's defeated the villain. Muscular survives the attack by amassing shield of muscle, but his prosthetic eye and tank top are destroyed.

Muscular compliments Izuku's attack, but notes it was far too obvious. Izuku tells him to stay back, but Muscular replies that attacking seems like the more fun option. Izuku demands to know why the League of Villains is attacking them, but Muscular states he doesn't care about any of that. He rummages through his pocket and grabs another eye. Muscular tells Izuku that he's done playing around because he realizes Izuku's actually pretty strong. He inserts his "serious eye" and covers his body in muscle fibers before attacking. Grabbing Kota and putting him on his back, Izuku manages to narrowly evade Muscular's attack.

Muscular attacks the pair again, with them once again evading him. Reprimanding himself on being over-excited, Muscular ends up getting his arm stuck in the cliffside floor. Izuku drops Kota after dodging and resolves to face Muscular on his own. The two clash, Izuku fighting against Muscular's mass with a Detroit Smash. Muscular easily pushes the youth back, taunting him about that attack being weaker than his previous attempt. Izuku holds his ground, however, and Muscular compliments him on his impressive feat. Muscular overwhelms him shortly afterward and pushes him into the ground, threatening to crush and kill him. Urging his young opponent to show his blood, the battle-hungry villain continues to press his attack. Suddenly, a deluge of water splashes him, the source of the attack coming from Kota, who demands Muscular let Izuku go.

Temporarily distracted from his struggle with Izuku, Muscular tells Kota to wait his turn, promising him that he is next. Muscular is shocked to feel Izuku pushing back against his crushing attack, frantically wondering how the youth is getting stronger. Pushed completely off of Izuku, Muscular is struck by his 1,000,000% Delaware Detroit Smash, breaking through the villain's muscles and smashing him into the cliffside, defeating him.

Some time afterward, Mr. Compress's message concerning the success of their mission is broadcast to all of the members of the Vanguard Action Squad, but the bloodthirsty villain is still unconscious. Later, after the rest of the Vanguard Action Squad escapes with the fruit of their labor, leaving the defeated members of their group behind, Muscular is arrested by the Police Force, heavily restrained and protesting vehemently.

He was imprisoned in Tartarus.

Paranormal Liberation War Arc
When an All For One controlled Tomura, and a group of Near High-End Nomu, raided Tartarus to break out All For One's original body, he utilized dual EMP attacks to shut down the power inside and outside the facility, unlocking nearly all the prison doors and exciting a riot. Muscular is among the freed prisoners, cheerfully attacking the guards attempting to push them back. He was present when the escaped All For One, alongside Tomura, offered him and his fellow prisoners freedom in exchange for pledging loyalty.

Tartarus Escapees Arc
About one month after escaping, Muscular starts rampaging throughout a nearly abandoned town that is in the middle of being evacuated under Ketsubutsu Academy High School third years Yo Shindo and Tatami Nakagame. Yo faces off against Muscular while Tatami takes care of the restless civilians. Yo manages to put up some of a fight, utilizing his Vibrate Quirk to resist against Muscular's attacks, but he proceeds to grow new muscle tendrils anyway and is prepared to slowly crush him to death. Right before he gets serious, a hooded Deku suddenly jumps in, saving Yo, and preparing to battle Muscular.

Muscular is quickly able to recognize Deku from their previous encounter and is joyfully excited by his reappearance, before calming himself down. He tells him he's not looking for revenge and just wants a good fight, placing a nearby rock into his empty eye socket. He then jumps toward the bottom of the building Deku is on, and throwing it into the air, screaming at his to fight him at full power like the last time.

Deku proceeds to bring Yo to safety, and uses his new arsenal of Quirks to get the upper hand on Muscular. He uses Float to hover above the city, as well as Smokescreen to cover the streets below and reduce Muscular's vision. He jumps up into the air to try and attack him, but Deku uses Danger Sense to dodge each of his incoming punches, and finally Blackwhip to grab a hold of him and keep him contained on the ground.

Muscular is annoyed at all the "cheap party tricks", and wants an actual fight with him. Deku questions where Tomura and All For One are, but he replies that he doesn't know or care and was told to go wild, berating Deku for talking and using all these gimmicks. Deku continues anyway to ask him why he rages like this, if he has any regrets, and if there is any other path he could go towards, even referring to him by his real name, Goto Imasuji. Muscular simply responds that he just wants to indulge in blood and violence and it's useless to try and sympathize with him.

As Muscular breaks free from Blackwhip and tries to fight back, Deku notes how his muscle tendons have started to unravel due to the previous attacks by Yo's vibrations, and that Muscular was purposefully keeping his distance because he recognized it too. Muscular screams in annoyance, as Deku comes at him with a Smash to the stomach, declaring that this is his full power, knocking the villain unconscious.

Deku quickly flies to the Daina Police Station, the closest one with iron maidens on hand, delivering them the still unconscious and tied up Muscular, to once again be arrested.