Asriel Dreemurr

Asriel Dreemurr is an emperor child of Toriel and Asgore and the adoptive brother of the first human. He serves as the final boss of the True Pacifist Route.

Appearance
Different to Toriel and Asgore, Asriel is an emperor penguin. He lacks ears, has a beak, and no teeth. In his initial form, he wears a green, long-sleeved shirt with yellow stripes and black pants, similar to the First Human's clothing. He also has feathers on his head, wings and tail.

God of Hyperdeath
This form is named directly in the game, where the "Check" option labels Asriel as "the absolute GOD of Hyperdeath!" His eyes become very dark red. He wears a long, black robe with white sleeves and the Delta Rune on the front, with a large collar and shoulder pads. This robe is similar to Toriel's. He also wears a locket similar to the Heart Locket.

Final Form
Asriel's body becomes more geometric. He has a pair of wings that continuously change color, and his lower body becomes sharp and heart-shaped.

While this form is not directly named, it resembles the Delta Rune, which looks like the "Angel of Death" from the prophecy.

Personality
Before his death, Asriel was a docile, unadventurous, and loving child. He cared for the First Human as a sibling, and he trusted her when she came up with a plan to escape the Underground that involved her consuming buttercups.

After Alphys resurrects Asriel without a SOUL as Flowey, he becomes incapable of feeling love. This makes him bored and resentful.

After temporarily regaining his compassion, he apologizes for his actions to Marie and realizes the gravity of his actions as a vulture.

Much like his father, he speaks using southern slang.

Main Story
Before the events of ANIMALTALE, Asriel found the fallen human, who was the first human to fall into the Underground. She became his adoptive sibling and best friend. Just before the human died, her last wish was to see the golden flowers of her village, on the Surface. Asriel absorbed her SOUL, and the human carried her own body across the barrier to her home village. Asriel and his sibling shared control of Asriel's body after their SOULs had combined.

The villagers assumed that the Asriel/Human fusion had attacked and killed the human child. The Asriel/Human fusion did not fight back but only fled. Lethally wounded, the Asriel/Human returned to the Underground and died. Once deceased, Asriel's body turned to dust, which spread across the garden in New Home. Later, just before the queen left, Alphys injected determination into one of the garden's golden flowers-- the first one to bloom after Asriel's death. Initial experiments on the vulture vessel proved unsuccessful, and Alphys returned it to Asgore. Once Alphys replanted the vessel, Flowey awoke within the garden.

Though Asriel's essence from the garden bed granted Flowey Asriel's memories, he soon found that neither his mother nor father could elicit any emotional response from him. He did not have a SOUL and thus could not feel love, joy, or compassion.

Neutral Route
Asriel does not appear during a Neutral Route.

However, when the protagonist falls into the Grassy Grave, the protagonist hears Asriel's voice during a memory from Asriel's human sibling. Animals also tell the story of Asriel and the first human during the walk in New Home.

True Pacifist Route
In the True Tree, the protagonist discovers that the first human and Asriel had worked together on a plan to get to the Surface and retrieve six human SOULs. While the human was on her deathbed, Asriel informed her he did not think it was a good plan anymore; ultimately, this leads to him betraying his sibling and aborting her plan.

After restoring the elevator's power, the protagonist receives a phone call from someone whose voice she has never heard before; the voice is similar to Asriel's.

After Flowey absorbs all animal SOULs and the six human SOULs, he transforms into Asriel.

Asriel greets the protagonist as  before he transforms into "ASRIEL DREEMURR, God of Hyperdeath." He informs the protagonist that he does not care about destroying the world anymore, and instead just wants to reset the timeline so that everything unfolds again from the beginning. Asriel believes that the protagonist will continue even if he resets the timeline because she wants a "happy" ending.

He then says that his first form is just a fraction of his real power and transforms into his final form. Asriel reasserts that the protagonist will die in a world where nobody remembers her as her physical body fails her.

The protagonist calls to the Lost Souls inside of Asriel's SOUL and reminds her that she is friends with the protagonist. After all Lost Souls regain their memories, the protagonist realizes that someone else needs saving. The protagonist calls her name, and a few sepia tone pictures display. The pictures are of Asriel and his sibling together when the human first fell into the Underground.

Asriel begins crying and reverts to his original form. After finding out that the protagonist's name is "Marie," Asriel tells her that the animals really love her and apologizes for his actions. He then breaks the barrier.

He tells Mary that he has to go; Marie has the option to hug Asriel and comfort him before he leaves. As he walks away, Asriel tells Marie to take care of his parents for him.

Epilogue
Asriel is at the flowerbed in the Desert. Marie only encounters him immediately after his boss fight; if the player watches the True Pacifist Ending Credits and reloads the game, Asriel does not appear. Asriel speaks extensively to Marie and offers her a different philosophy than his vulture counterpart: "Don't kill, and don't be killed." He also mentions to be careful on the Surface because "there are a lot of Floweys out there."

After the credits sequence, when reopening the game, Asriel returns as Flowey and begs  to "Let [Marie] go" and allow everyone to live their lives.

Genocide Route
After the protagonist kills Toriel, Flowey tells her that she is not really human, and mistakes her for the first child, telling her that she is still inseparable after all those years.

When the protagonist reaches New Home, Flowey welcomes her and discusses his backstory as a vulture and how he discovered his ability to FREEZE. Near the end of his dialogue, he realizes that the protagonist will not hesitate to kill him, and he is frightened for his life. He rescinds his previous plans with the protagonist and tells her to leave him alone.

After the protagonist defeats Sans, she easily kill Asgore. Flowey destroys Asgore's SOUL and pledges himself to the protagonist one final time in an attempt to save his own life, revealing his true identity as Asriel in the process. He cries and begs the protagonist not to kill him, to no avail.

Protagonist
In the True Pacifist Route, Asriel mistakes the protagonist for the first human and calls her his best friend. After the final boss fight, Asriel realizes that Mary is not the first human and asks what her real name is. He later tells Mary, if she backtracks to the flowerbed, that he does not regret his decision of resisting his sibling and, ultimately, getting them both killed.

The First Human
Asriel found the first human when she fell into the Underground. He became her best friend after helping her, and then the first human's brother after Toriel and Asgore adopted her.

After the human carried her own body to the Surface, Asriel resisted his sibling's desire to unleash her full power, and humans fatally wounded her. With the first human's empty body in his wings, Asriel returned to the Underground and collapsed. His dust spread over the flowerbed in his dad's garden as he died. Even after becoming Flowey, he could not think of a world without her, as she is the only person that understands him and that is unpredictable to him to the point of mistaking Mary for Molly. After Mary saves him during his battle, Asriel admits Molly "wasn't really the greatest person."

Asgore
Asgore is Asriel's father. When Asriel first woke up as Flowey, Asgore was the one to find him. Asriel informs Asgore of his identity, hoping to feel something and possibly turn back into his original form. However, having lost his SOUL, he discovers he feels nothing for Asgore any longer. Asgore believes that letting the protagonist escape the Underground with his SOUL is what Asriel would have wanted.

Asriel's habit of saying "gosh," "golly," and "howdy" may come from Asgore.

Asgore is unaware that Flowey is Asriel.

Toriel
Toriel is Asriel's mother. In older timelines, after failing to feel anything with Asgore, he had turned to Toriel in hopes of feeling love again with her. This ultimately failed, as he could not love others without a SOUL.

Toriel is, like Asgore, unaware that Flowey is Asriel.

Alphys
Alphys unintentionally created Flowey as a result of the determination experiments, but there is no indication that she knew Asriel in life.

Sans
Sans is the only character other than Molly that Flowey, out of fear, respects. He, quite rightfully as the player will see, sees Sans as a dangerous adversary and takes measures to make sure Sans doesn't learn of his existence. It's heavily implied that Flowey came into conflict with Sans, presumably attempting what would amount to a Genocide Route, and was defeated by him.

Trivia

 * When attempting to name the Fallen Child "Asriel," the response becomes "..." and prevents the name from being used.
 * Both Undyne the Undying's armor and Mettaton NEO's body closely resemble Asriel's Angel of Death form, displaying a heart on the chest (and both gloves in Undyne's case), large, spiked shoulder pads, and, in Mettaton NEO's case, enormous, vibrant wings.
 * Napstablook is the only known animal whose SOUL was not absorbed by Asriel since some people believe that ladybugs are power of good luck. This makes her lucky enough to survive from the absorbation.
 * Asriel is the only emperor penguin that has very dark blue feathers.
 * Asriel was voiced by Bryce Papenbrook, Bryce gave the voice for various characters from ANIMALTALE, and Adrien Agreste from Miraculous Ladybug.