Asriel Dreemurr

Asriel Dreemurr is the biological child of Toriel and Asgore, the adoptive brother of Chara, Flowey's former self, and the final boss of the True Pacifist route.

Appearance
Asriel has a similar appearance to his parents, with long ears, a snout, and visible fangs. In his child form, he wears a striped long-sleeved shirt and pants.

In his first transformation, the sclera of his eyes becomes black, his fangs become longer, and he grows a pair of horns. In this form, he wears a long robe with the Delta Rune on it, similar to Toriel's. He also appears to be wearing a locket similar to the Heart Locket.

In his final form, his body is reminiscent of the Delta Rune. His body becomes more geometric, his horns longer and his teeth sharper. He grows a pair of wings that constantly change color, and his lower body becomes a sharp and heart-shaped.

Personality
Before dying, Asriel was a docile, unadventurous and loving child who loved jokes. After being brought to life again without a SOUL as Flowey, however, he became incapable of feeling joy, leaving him bored and resentful. After being fought, he apologizes for his actions, and eventually becomes a flower again, but this time with compassion; as he asks the player not to reset the timeline.

Background
Before the events of Undertale, Asriel found the first human that fell into the Underground, who became his adoptive sibling and best friend. When the human died, their last wish was to see the golden flowers of their village, on the Surface. Asriel absorbed their soul and carried their body across the barrier to their village, whose inhabitants assumed Asriel had killed the human and attacked him. Asriel, lethally wounded, returned to the Underground where he died, his dust spread across the garden. Later, Alphys, as part of her experiments, injected determination on the first golden flower that bloomed on that garden ever since. This brought the flower to life; with the flower as a vessel for his essence, Asriel was reborn into Flowey, although he had no soul in this state. Unable to feel love or joy, Flowey took his own life. However, through the determination that Alphys had injected, Flowey obtained the power to SAVE, and Flowey found himself returned to where he was first brought to life. Using that power, Flowey first decided to help the Underground, but became bored, and began killing monsters. After seeing and doing everything he could, he became apathetic.

All Routes
When the protagonist falls into the Underground, their determination outweighs Flowey's, taking away his ability to SAVE. Flowey first tells the protagonist that in this world, "it's kill or be killed", and tries to kill them in an attempt to steal their SOUL, but is stopped by Toriel. He appears again at the end of the Ruins after fighting Toriel.

Later on, when the protagonist falls into the Garbage Dump, Asriel appears in a memory of them.

True Pacifist Route
In the True Lab, after restoring the elevator's power, the protagonist receives a phone call from what is believed to be Asriel, mistaking the protagonist for the first human.

Asriel appears again as Flowey when the protagonist and their friends come together after they try to fight with Asgore. Flowey absorbs the soul of every monster in the Underground, and transforms back into Asriel. Asriel tells the protagonist that he doesn't care about destroying the world anymore, and instead just wishes to reset the timeline so everything unfolds again from the beginning.

He then says that his first form is just a fraction of his real power, and transforms into his final form. Asriel tells the protagonist that they will die in a world where no one remembers them. The protagonist calls the Lost Souls inside Asriel, and Asriel tells them that the reason he keeps fighting to keep them around is because they're the only one that understands him and the only one who is fun to play with anymore. Asriel begins crying and reverts to his original form as a child. He then realizes that the protagonist was not the first human, and that their name is Frisk. He tells Frisk how much other monsters love them, and apologizes to Frisk for hurting them, and proceeds to destroy the barrier that kept the Underground apart from the surface.

Epilogue
During the epilogue, he is seen at the flowerbed where Frisk fell at the beginning of the game. He tells about the first human's cruelness.

After the credits sequence, Asriel returns as Flowey, begging the player not to reset the game, as that would erase all of Frisk and the monsters' happiness.

Genocide Route
After the protagonist kills Toriel, Flowey tells them that theyre not really human, and mistakes them for the first child, telling them that they are still inseparable after all those years.

When the protagonist reaches New Home, Flowey tells the protagonist everything about his life as a flower, his shock when he woke up in the garden without limbs, his lack of feelings and compassion, his suicide, and how he got the ability to SAVE. Flowey explains that at first, he used his SAVE ability for good. After a while however, everyone became predictable, and that's when he decided to kill others. Flowey dismisses his reason for killing people as an excuse, assuring the protagonist that they themselves know how liberating it is to act that way, and calling out those who "want to see it, but are too weak to do it themselves". He then says that even that bores him, and that he has seen and done everything that can be done in the world. Flowey tells the protagonist how unique they are, and how their determination is greater than his. He tells them how tired he is of everything and how much he wants to show humanity that despite everything, this world is still kill-or-be-killed. He then realizes that the protagonist won't hesitate to murder him as well, and escapes to Asgore, warning him about the protagonist.

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

In Battle

 * See Asriel Dreemurr/In Battle

Player
Asriel speaks to the player directly after the True Pacifist Ending Credits, begging them to never play the game again, for the sake of everyone's happiness.

Frisk
At the beginning, Flowey attempts to kill Frisk and steal their soul. In the Neutral Route, Asriel as Photoshop Flowey fights Frisk and intends to kill them as many times as possible.

In the Pacifist Route, Asriel mistakes Frisk for the first human, and begs them to let him win so he can't be alone anymore. He is the first to realize Frisk isn't the first human.

Chara
Asriel was the first to find Chara (the first human) when they fell into the Underground. He became Chara's brother and his best friend. In the Pacifist Route, he describes them as being the only one who understands him. In the Genocide Route, Asriel describes their relationship as inseparable. At the end of the route however, he is murdered by Chara.

Asgore
Asgore is Asriel's father. When Asriel woke up as Flowey, Asgore was the first to find him. Asgore was initially aware of Flowey's true identity at this point, however he seems unaware of it in the Genocide Route, likely due to Flowey's own experimentation with the timeline.

Flowey's habit of saying "gosh" and "golly" may come from Asgore.

Toriel
Toriel is Asriel's mother. When he woke up as flowey, he turned to Toriel in hopes of feeling love again with her. Toriel is unaware of the fact that Flowey is Asriel.

Alphys
Flowey is the result of Alphys's experiments, his vessel being the first Golden Flower that sprouted from Asriel's dust, injected with Determination.

Trivia

 * Asriel Dreemurr is an anagram of Serial Murderer.
 * His name is likely a mix of Asgore and Toriel.
 * Asriel might be derived from "Azrael", the Angel of Death in Judaism and Islam.
 * Dreemurr is similar in pronunciation to "dreamer", referenced by the ACT "dream" in his battle, and his leitmotif name "Hopes and Dreams".
 * When attempting to name the Fallen Child "Asriel", the response becomes "..." and prevents the name from being used.
 * Undyne the Undying's armor closely resembles Asriel's God of Hyperdeath form.
 * Napstablook is the only monster whose soul isn't absorbed by Asriel during the fight against him.
 * Its impossible to damage Asriel or any of the Lost Souls. Any attempt to FIGHT either will result in a MISS.
 * During the fight against Asriel, if the protagonist hasn't saved before, dialog will change to "SAVING over ASRIEL's file seems like the only way to defeat him..." "But having never SAVED before, you lack the power to do it."
 * Asriel's attacks "Star Blazing" and "Galacta Blazing" strongly resemble PK Starstorm.