Goto Imasuji/Relationships

Mustard
Mustard and Muscular have opposing views on strategic planning. Mustard believes that it's best to wait for the right time while Muscular believes that it's best to just attack now and worry about the details later.

Water Hose
Muscular faced Kota's parents when he attacked a village two years before the beginning of the story. He killed both at the cost of losing his left eye in the confrontation. Besides this, he doesn't hold a grudge against Kota's parents for the loss of his eye. In a twisted way, he respects them for facing him. He wanted to kill people and they wanted to stop him, and he accepts all the results as they played out. However, he considers their deaths to be only their fault alone, because it was their actions that got them killed, calling them pathetic for trying to act as heroes and think they could stop him even though they were incapable.

Kota Izumi
Muscular met Kota by chance when the Vanguard Action Squad attacked the Quirk Training Camp to kidnap Katsuki Bakugo. At first, he thinks he is just a random kid and attacks him for his hat, while Kota recognizes him as the killer of his parents. Then, Muscular is surprised to know that Kota is the son of the Water Hose duo and considers their meeting to be fate. Kota blames the villain for killing his parent. Muscular criticizes Kota for trying to put the fault on him. He responds to Kota that he does not hold any grudges towards them for ripping off his left eye but it was the actions of his parents that got them killed, not his own. Their deaths were the result of everyone involved doing their best, and the only real tragedy is when people don't deliver on promises they make

Muscular prepares to strike Kota to kill him, but Izuku intervenes. Kota watched as Izuku continued to fight Muscular and soon saved Izuku from a fatal blow, distracting him long enough for Izuku to regain strength and defeat the villain.

Izuku Midoriya
Muscular meets Izuku for the first time when the latter saves Kota Izumi from being killed by him. Muscular expresses surprise upon recognizing Izuku and told him that he has orders to kill him while Izuku resolves to face him alone. In their fight, Muscular takes glee in tormenting Izuku who pushed his body and Quirks to his limits. Izuku was also horrified to learn from Muscular that Katsuki is also a target for the League and to learn Muscular is the person who killed Kota's parents, which installed his dislike of Quirks and the hero society.

Overcoming his internal struggle, Izuku fought back harder and pushed him back, leaving Muscular surprised and impressed. Muscular took the time to fight all out and overpowered Izuku but he is saved by Kota who distracts him enough for Izuku to use One For All at 1,000,000%. With an amplified Delaware Smash, Izuku is able to break apart Muscular's muscle fibers and send him flying deep into the rock wall, finally defeating the villain.

Several months later, after Muscular is broken out of Tartarus and nearly kills Yo Shindo, Izuku rushes to save him; upon recognizing his voice, Muscular smiles, appearing eager to fight Izuku a second time.