Forest Training Camp Arc
In a flashback, Muscular is seen on a news report that was being watched by Kouta Izumi that speaks of how Muscular murdered Kouta's parents, a pro hero duo known as the Water Hose.
Two years later, Muscular is seen wearing a mask and a hooded robe meeting with the other members of Vanguard Action Squad, preparing for their attack on the U.A Forest Training Camp while barely holding back his blood thirsty thoughts.
He is then seen stalking Kouta in the cliff side where Kouta usually hides from the other camp members. Kouta immediately recognizes him as the man who murdered his parents after Muscular removed his mask and hooded robe. Muscular proceeds to attack Kouta only for Izuku Midoriya to appear and rescue Kota before Muscular was able to crush the boy. Realizing that his cellphone was broken because of the attack, Midoriya is forced to protect Kouta on his own without any backup. Muscular easily overpowers Midoriya in his Full Cowl State, forcing the boy to use his quirk at full power and injuring himself. However, his attacks barely injure Muscular but decides to take Midoriya seriously before augmenting his own power with an eye piece. Midoriya is nearly overwhelmed and killed by Muscular while holding him back and ordering Kouta to run but is saved when Muscular is distracted by Kouta using his quirk on Muscular and Midoriya is able to knock Muscular unconscious with his strongest punch.
Muscular's quirk allows him to augment his muscle density, capable of making his muscles durable enough to the point of taking full powered punches from Midoriya, who was capable of leveling buildings with his strongest attacks, with little damage and strong enough to make him hit harder than a regular person can.