Asriel Dreemurr

Asriel Dreemurr (typically /ˈæzriəl ˈdriːmər/) is the biological 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
Similar to Toriel and Asgore, Asriel has long ears, a snout, and visible fangs. 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 a small wisp of fur on his head and lacks horns.

God of Hyperdeath
This form is named directly in the game, where the "Check" option labels Asriel as "the absolute GOD of Hyperdeath!" The sclerae of his eyes become black, his fangs become longer, and he now has a pair of horns. He wears a long, dark 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, his horns are longer, and his teeth get sharper. 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 them when they came up with a plan to escape the Underground that involved both of them 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 Frisk and realizes the gravity of his actions as a flower.

Much like his father, Asgore, he speaks using southern slang.

Main Story
Before the events of Undertale, Asriel found the fallen human, who was the first human to fall into the Underground. They became his adoptive sibling and best friend. Just before the human died, their last wish was to see the golden flowers of their village, on the Surface. Asriel absorbed their SOUL, and the human carried their own body across the barrier to their 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 flower vessel proved unsuccessful, and Alphys returned it to Asgore. Not long after being replanted, Asriel awoke within the garden, reincarnated as Flowey.

Though the injection of determination granted Flowey the will to live, 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 Garbage Dump, the protagonist hears Asriel's sound bite during a memory from Asriel's human sibling. Monsters also tell the story of Asriel and the first human during the walk in New Home.

True Pacifist Route
In the True Lab, 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 their deathbed, Asriel informed them he did not think it was a good plan anymore; ultimately, this leads to him betraying his sibling and aborting their plan.

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

After Flowey absorbs the six human SOULs and most of the monster SOULS, he transforms into Asriel.

Asriel greets the protagonist as &lt;Name> 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 they want 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 them as their physical body fails them.

The protagonist calls to the Lost Souls inside of Asriel's SOUL and reminds them that they are friends with the protagonist. After all Lost Souls regain their memories, the protagonist realizes that someone else needs saving. The protagonist calls their 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 "Frisk," Asriel tells them that the monsters really love them and apologizes for his actions. He then breaks the barrier with the power of the SOULs.

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

Epilogue
Asriel can be found at the flowerbed in the Ruins. Frisk 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 Frisk and offers them a different philosophy than his flower 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 &lt;Name> to "Let [Frisk] go" and allow everyone to live their lives.

Genocide Route
After the protagonist kills Toriel, Flowey tells them that they are 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 welcomes them and discusses his backstory as a flower and how he discovered his ability to SAVE. 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 them to leave him alone.

After the protagonist defeats Sans, they 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 them his best friend. After the final boss fight, Asriel realizes that Frisk is not the first human and asks what their real name is. He later tells Frisk, if they backtrack 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 they fell into the Underground. He became their best friend after helping them, and then the first human's brother after Toriel and Asgore adopted them.

After the human carried their own body to the Surface, Asriel decided to not act on his sibling's desire to unleash their full power to protect themselves, and humans fatally wounded them. With the first human's empty body in his arms, 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 them, as they are the only person that understands him and that is unpredictable to him to the point of mistaking Frisk for Chara. After Frisk saves him during his battle, Asriel admits Chara "wasn't really the greatest person." This shows his loss of his idealization of Chara, but doesn't necessarily mean that Chara was ever mean towards him.

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" came 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, Flowey had turned to Toriel in hopes of feeling love again with her. This ultimately failed, as he could not love others without a SOUL.

Like Asgore, Toriel is 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 Chara 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

 * Asriel's general appearance and green shirt resembles the race of "Mimigas" from the indie game Cave Story.
 * Asriel's name has several possible meanings or origins.
 * Asriel's name is a combination of his parents' names, Asgore, and Toriel, likely stemming from Asgore being terrible at naming things. An interesting thing to note is that this follows Anglo-Saxon naming traditions, from where the English language imported names like Albert, Edgar, or Wilfred.
 * "Asriel" might be derived from "Azrael," the angel of death in the Hebrew Bible. Gerson mentions an Angel of Death in his explanation of the Delta Rune prophecy.
 * "Asriel" is an anagram of "Israel", the country where Judaism is founded. Azrael is the angel of death in Judaism.
 * "Asriel Dreemurr" is an anagram of "serial murderer," to which the character's actions somewhat represent. This may, however, simply be coincidence.
 * "Asriel" is an anagram of "Ralsei," which is the name of a character from Deltarune, a game Toby Fox also created.
 * "Dreemurr" is similar in pronunciation to "Dreamer," alluded by the ACT "dream" in his battle, and his leitmotif "Hopes and Dreams."
 * "Asriel" is also a name of Hebrew origin that means "Helped by God." This might be a reference to Asriel being miraculously saved and turned to a flower after dying or helping Frisk by breaking the barrier with his god-like powers. This name appears several times in the Tanakh, the equivalent to the Christian Old Testament.
 * His name and appearance could have also been inspired by Sariel, from the game Touhou: Highly Responsive to Prayers. Their names are similar, and this character's outfit resembles the God of Hyperdeath's (it also bears a symbol similar to the Delta Rune). Sariel is also an angel of death.
 * His name may come from Lord Asriel, a character from the His Dark Materials series. Lord Asriel from the series has "enormous determination and willpower" which correlates with the theme of Undertale: Determination.
 * 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 monster whose SOUL was not absorbed by Asriel.
 * If the game is hacked, it is possible to spare Asriel Dreemurr during his boss fight, and the protagonist will be left on overworld where they left off: in front of Asgore, with the six souls. Nothing will happen, and the protagonist can walk around and even above the barrier.
 * It is possible that Asriel inherited his father's trait of being bad at coming up with names, as when he is in his flower form he names himself "Flowey."