Master of the Waves

Beady, sunken eyes and a pointed nose sit within a head that is essentially an entire squid, body and tentacles both, interrupted only by a jawline defined enough to keep the shape from reading as pure animal.

Fins jut from his wrists, and his hands and feet are webbed. The build underneath is fairly lean, with the abdominal, chest, bicep, and forearm muscles apparently left exposed. Tight dark trousers are the only thing he wears.

Land-based society refused to do as he asked, and he has hated it since. Aggression is his default setting.

The sea creatures under his command are instruments rather than companions, and their welfare interests him only where it advances his claim to rule every wave.

Shoto Todoroki, Kyoka Jiro, and Koji Koda look into the disappearance of Takoba Aquarium's animals and turn up sea anemones that Koji cannot reach with Anivoice. The failure repeats with every fish they find afterward.

Then the water churns. Marine life surges up, and a squid-like man rises with it, pulling the injured creatures back under his command and giving his name: the Master of the Waves. Since the surface world will not obey him, he announces, he intends to dominate all sea life and rule the waves outright.

He attacks the three students. Koji's Quirk is still jammed by the villain's interference, and he goes into the water while sea animals swarm Kyoka and Shoto on land. Submerged and surrounded, Koji weighs whether to harm the creatures in order to free himself, and cannot do it. He reaches past the interference instead and calls a whale, which slams down onto the villain, breaks his grip on every creature present, and ends the fight. His arrest is presumed to have followed shortly.

The unnamed Quirk behind all of it commands any sea life nearby. No ceiling has been established, though a whale falls within its reach.