Evacuate? Nonsense. As if I would allow any of those monkeys to escape.
~ Sea King

The Sea King is a major villain from One Punch Man. He appears as a large green merman wearing only a crown, cape and a speedo. He is the king of the Seafolk, who have invaded the land to kill the humans and claim the land for themselves. Sea King believes that since live began on the sea and he is the king of the sea, this puts him on the top the food chain and thus he has the right to claim the land for himself.

Sea King first shows up after all the Seafolk are killed by the A-class hero Stinger, punching him and leaving him unconscious. Inazuma Max, another A-class hero who had been watching is also attacked, attempting to fight the monster but also ending defeated after his attacks fail to even scratch him. Sea King is then confronted by the S-class hero Puri-Puri Prisoner, who had just escaped from prison along with Sonic. Puri-Puri and Sea King trade punches, but Puri-Puri is visibly hurt harder, so he unleashes his Angel Mode, attacking Sea King with a barrage of punches. The attack doesn't make much damage on Sea King, who counterattacks with a punch barrage of his own, beating Puri-Puri Prisoner.

Sonic decides to challenge Sea King, confident that he wouldn't lose. He effortlessly dodges all of Sea King's attacks, but as it starts raining Sea King grows bigger, becoming stronger and faster. Unable to cause damage on Sea King, Sonic runs away, so Sea King goes after a shelter dome where people had taken refuge. As he busts through the roof of the dome, some C and B-class heroes who were there attempt to stop him, but they're all easily defeated. Genos arrives at the place and confronts the monster, damaging him but losing an arm after lowering his guard. Genos loses the fight after protecting a girl from Sea King's acid spit, but before he's finished off the C-class hero License-less Rider stops him. Sea King effortlessly beats him, but License-less Rider manages to buy enough time for Saitama to arrive at the place. Sea King punches Saitama, but his attack has no effect on him. After some bragging, Sea King attacks again, but ends killed with a single punch from Saitama.