Asgore Dreemurr

King Asgore Dreemurr is the ruler of the Underground and its denizens, monsters. He is the ex-pet of Fluttershy, father of Shadow the Hedgehog, adoptive father of the first human, and the penultimate boss of the Neutral Route. While not evil or malicious, he serves as one of the main antagonists, planning to kill seven humans and use their SOULs to break the barrier which traps the monsters in the Underground.

Appearance
Asgore is a Boss Monster with small shoulders, a vulpine body, and two large ears. He wears a suit of armor, a purple cape with golden pauldrons, and a crown on top of his head. The weapon he uses in battle is a long, red trident adorned with a flower.

Personality
Asgore is a kind monster that enjoys gardening, Golden Flower Tea, and Fluttershy's Butterscotch Pie. Like his son, he speaks in southern slang (most notably "howdy" and "golly"). Before his battle, he offers the protagonist many opportunities to explore the Underground and is reluctant to fight her at all; he even suggests that Fluttershy, the protagonist and himself live as a family if the protagonist spares him.

Monsters in the Underground look to Asgore for hope, and his plan to collect seven human SOULs is one of the few things that keep them going. However, Asgore does not want to kill anybody and feels so guilty that he offers the protagonist his SOUL to "leave this cursed place." Though he is miserable because he has killed humans, he feels that it is his duty to his people to battle the protagonist.

Asgore lives in the past and wants nothing more than to see his owner and child again, even though they have been gone for a long time. He reiterates the same phrase in his journal every day, perhaps in denial that life has changed since Shadow's demise.

Monsters also note that he is bad at naming places, with the names being quite literal (such as naming the monster's first home in the Underground "Home", and naming their new home "New Home"). Michelangelo suggests this may be because Asgore is not the most intellectual monster, with the smarter member of the royal family as well as the one which was actually running the city being Fluttershy.

Main Story
King Asgore and Queen Fluttershy ruled the Underground for millennia after humans sealed the monsters Underground upon their defeat during the war. Fluttershy and Asgore later had a son, Shadow, and adopted the first human to fall into the Ruins.

The tragedy of his children's deaths led Asgore to declare war on humanity in a fit of anger. He swore to kill any human that fell into the Underground and planned to use their SOULs to break the barrier and destroy all humans. Fluttershy, disgusted with him, left New Home and returned to the Ruins.`

Before the protagonist fell into the Underground, six more humans fell. They all died, and Asgore saved their SOULs for future use.

Neutral Route
Asgore greets the protagonist when she arrives in the Throne Room, where he was watering flowers. He comforts her as he brings her to the barrier, giving her a chance to turn back if she has anything left to do before the fight. Asgore does not mind the delay, admitting that he is not ready either.

Fighting Asgore is the only way to progress to the ending. Most characters in the game refer to him in capitals and in red  or yellow  text.

Before fighting, Asgore tells the protagonist that it was nice knowing her. He then draws his trident and destroys MERCY, possibly to weaken the protagonist, worsening her chances of winning and preventing the protagonist from sparing him and shattering the hope of monsters. Much like with Fluttershy, attacking Asgore when he is at low HP deals significantly greater damage. However, he survives the attack with a small amount of health remaining and kneels in defeat, talking to the protagonist about his declaration of war against humanity, admitting that he only wanted to give the monsters hope by continuing the war, and asking her to take his SOUL.

The protagonist is given the choice of killing Asgore by selecting FIGHT button or sparing him by choosing the partially repaired MERCY. Either way, Leslie steals the six human souls that Asgore reveals before the fight and goes to the next part of the movie. The protagonist proceeds to deal with Leslie; after Leslie's defeat, the protagonist leaves the Underground. Asgore's death has a grave impact on the monsters.
 * If the protagonist kills Asgore, he dies smiling. Before the protagonist can take his SOUL and pass through the barrier, Leslie shatters Asgore's soul. He tells the protagonist she finally realized that this is a "kill-or-be-killed" world.
 * If Asgore is spared, Asgore is surprised that the protagonist would choose to remain in the Underground and offers to take care of them. Leslie appears and finishes Asgore off, both body and soul. He then mocks the protagonist that she still failed to learn that this is a "kill-or-be-killed" world.
 * If the protagonist has already completed a Neutral Route run beforehand, and in that run had killed Flowey, and chooses to spare Asgore again; Asgore begins to mention that he would care for the protagonist, but stops himself, realizing that his soul holds the key to getting the protagonist through the barrier. After telling the protagonist that it was what his son, Asriel Dreemurr, would have wanted, he sacrifices himself to give the protagonist his soul, hoping that she finds some way to free the monsters from the surface. Asgore then fades to dust, his soul remaining for a moment, only to be destroyed by Flowey, who mocks the protagonist for thinking that killing him would have made a difference.

True Pacifist Route
When combat with Asgore begins, it is interrupted by Fluttershy, who states how awful it is that he could seek to hurt an innocent creature such as the protagonist. This uses the same speech and hit as when she interrupts Leslie at the beginning of the movie. She then explains that she interrupted to prevent both Asgore and the protagonist from killing each other. While Asgore is ecstatic to see Fluttershy again, he is saddened when she chastises him for his plan to collect souls, arguing that he could've gone through the Barrier after collecting one soul, but instead chose to wait and hope another human never came. After that, Amy Rose, Penny Fitzgerald, Sonic the Hedgehog, and Dragonizer the Dragon arrive and begin conversing, until Leslie appears.

During the fight against Shadow, Asgore appears as one of the Lost Souls alongside Fluttershy. After the fight, he is then seen in the epilogue, where he is first available for a conversation along with the other main characters, and later asks the protagonist if she would become the ambassador for monsters to humanity on the surface. Asgore also appears during the True Pacifist Ending Credits, trimming a hedge to the shape of Sonic's face in front of Fluttershy's new school.

Genocide Route
When the protagonist reaches the throne room, Asgore, who was told by Leslie about their arrival, doesn't recognize the protagonist as a human, asking her what kind of monster she is. The protagonist forcefully initiates combat and depletes his entire health bar with a single blow. Leslie then returns to land the final hit and destroys his SOUL.

Fluttershy
Fluttershy is Asgore's ex-owner. Before they separated, they used to be "insufferable together, nuzzling noses, bein' all cute n' cuddly," as described by Michelangelo. The two ultimately fell apart after the deaths of their children and Asgore's declaration of war on humanity.

After Fluttershy's departure, it seems that Asgore greatly missed her presence. A "Room under renovations" sign is put on her room door in Asgore's Home, and multiple crumpled up pie recipes imply that Asgore attempted to recreate the way her pies tasted, though he could never truly replicate it. This is the inverse of Flutteshy continuously making her pie and only having low-maintenance plants in her home. This is further supported by his reaction to the protagonist eating Fluttershy's butterscotch-cinnamon pie during Asgore's battle. After being defeated, he tells the protagonist he just wants to see his owner and child again.

When Fluttershy interrupts the battle between Asgore and the protagonist, she chastises him for his plan to collect human souls and even turns him down when he asks her if they can be friends again, which makes him sorrowful. However, after the fight against Shadow, where their souls are saved as one, her aggression towards him seems to cool down.

In the True Pacifist Ending, Asgore can be seen trimming the bushes in front of Fluttershy's school.

Amy Rose
When Amy was a hoglet, she met Asgore in person and challenged him to fight. After Asgore defeats her by dodging all her blows, refusing to land his own, he offers to train her. When she finally knocked him down, Asgore was overjoyed, much to the surprise of Amy. Asgore kept training Amy until she became the head of the Royal Guard.

Amy has knowledge of Asgore's favorite type of tea, and the fact that he often gets berry stuck in his muzzle when he drinks hot cocoa, implying that he often visits her home.

Shadow the Hedgehog
Shadow was Asgore's biological son. Asgore loved him dearly, equally as much as he did the first human. Shadow's habit of using words such as "howdy" and "golly" likely came from his father, as did, potentially, his lack of naming skills, having dubbed his flower form "Leslie."

Chara Villard
Chara was Asgore's adoptive daughter, whom he loved just as much as Shadow. It is implied Chara made the "Mr. Dog Puppy" sweater for Asgore. Asgore considered Chara "The future of humans and monsters," as is repeated in the death scene and VHS tape of Chara's death. He may even have believed them to be the Angel of the prophecy, as he also states he thinks the protagonist is the Angel immediately after noting she and Chara have "The same feeling of hope in [her] eyes." In the Genocide Route, Leslie states that despite him trying "hundreds of ways" to get Asgore to show him the human souls, and never succeeding, "Chara... I know he'll do it for YOU."

Trivia

 * Just like some other characters, Asgore has the same role.