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. 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 is currently imprisoned in Tartarus.