Asgore is the king of the Underground, a land under the earth where monsters were sealed in after a war with the humans for the surface. One day, a human named Chara fell into the Underground, so Asgore and his wife Toriel adopted them as a sibling for their son Asriel. Unfortunately, Chara died after eating poisoned flowers, and Asriel died as well after absorbing their soul and attempting to return their body to the surface, where he was attacked and beaten brutally. This drove Asgore into a rage. He said that any human who fell into the Underground would be killed, their souls taken in hopes that he could get seven of them, break the barrier between the surface and the Underground with their power, and invade the surface to get revenge on the humans. Despite all that, Asgore feels guilty for every human he has killed, and tries to be pleasant even before fighting one to the death.
Fire Magic: Asgore and Toriel's main method of ranged attacks is the ability to control and summon fire at will. With this magic, Asgore can call waves of fire, expanding rings of fireballs, rains of fireballs, and spiraling pillars of flames.
Blue and Orange Trident Attack: With this attack, Asgore's eyes glow in a pattern of orange or blue. This'll tell the order he swings his trident in, letting the player know if they need to move through orange attacks, or stay still for blue.
Trident: Asgore's signature weapon is a massive red trident. By swinging the trident, Asgore can leave behind trails of blue or orange magic. Blue trails only hit stationary opponents, while orange trails hit moving opponents.
Holds back a lot: Despite his high strength, Asgore often holds back due to the guilt he feels from killing people. In his battle with Frisk, he'd hold back until Frisk was at 1 HP to try and avoid killing him.
Doesn't like killing: Asgore is very uncomfortable with killing, and will try to avoid it at all costs.
Reminders of his wife will make him hold back more: Since Toriel left him for what he did after Asriel died, reminding Asgore of her, such as showing him a pie she made, will make him hold back more.