Asriel Dreemurr

Asriel Dreemurr is a monster, the child of Toriel and Asgore, and the brother of the Fallen Child (Chara).

S POILER W ARNING : SUPER Serious spoilers inbound

Appearance
Asriel takes the appearance of a humanoid goat, similar to his parents, with 3 forms:
 * In his real original form, he's a simple child wearing a long-sleeved shirt and pants, with small endearing fangs.
 * After absorbing the souls of six humans and countless monsters, the whites of his eyes turn black, his fangs grow, and he grows horns and wears a long robe with the Delta Rune on it, similar to his mother Toriel's outfit.
 * When using his full power, his horns grow, his teeth all turn sharp, he grows large draconian red-and-green wings, and his entire lower body morphs to appear sharp, demonic, and mechanical - everything below his waist becomes one sharp down-pointing heart shape.

Personality
According to audio logs found in the True Laboratory, Asriel was "a bit of a crybaby", somewhat cowardly and docile, and a kind and loving child who loved jokes and pranks.

Unfortunately, a lab experiment restores him to life (albiet without his soul) as the heartless Flowey. Left only with cunningness and determination, Asriel eventually starts tormenting the monsters around him for fun, as he is unable to take joy in seeing even his parents happy.

Growing bored of this, he sets sights on the human Frisk, who reminds him of the Fallen Human - the Fallen Human being his sibling and best friend, and the only being who understood him. Remembering his own loneliness, Asriel seeks to vicariously trap Frisk in an endless time loop to toy with them.

As Asriel absorbs the monster souls of the entire Underground, Asriel regains his ability to feel emotions. Finally, he apologizes for his actions, and even despite the barrier being opened, he remains underground as he will soon turn back into a soulless golden flower.

Main Story
Upon the fallen human stumbling into the Underground, Toriel and Asgore adopt them as a second child to Asriel. When the fallen human dies, Asriel absorbs their soul and fulfills their last wish by taking their body back to their village. However, the humans of the village mistakenly assume that Asriel himself had killed the child. Through the power of determination, Asriel is able to place the first human's body in a bed of golden flowers in the center of their home village, endure relentless attacks, and make it back to the underground before dying. His ashes fell onto the garden, coating a golden flower there. As part of her experiments with determination, Alphys injected the substance into the golden flower. This process caused the deceased Asriel to be reborn into Flowey, but lacked his soul and thus the ability to love or feel compassion.

Pacifist Route
After absorbing all six of the human souls Asgore had gathered and ambushing everyone present in the final confrontation, Flowey uses his new power to absorb the souls of all the monsters as present as well, giving him equivalent power to seven human souls and becoming the god that Asgore had planned to become. His primary use of his power was to grant himself his old form, again becoming a small child as he appeared while he was still alive; his emotions had been almost completely repressed by the flower he inhabited due to existing as a creature with a mind, but no soul. He used his incredible power to attempt to kill the only thing still standing in his way-- the player character-- and perform a massive reset, which would let him relive his entire life up to that point. However, he repeatedly referred to the player character by the name chosen at the game's beginning, which was actually the name of the first child. Asriel took a new form to combat the player character with, appearing as himself at a much older age with markings on his fur and developed horns. The player character's determination won out, but Asriel revealed he had only been using a small fraction of the power he possessed; taking another, much larger form, Asriel attacked again, but the player called out to all the trapped souls of his friends within Asriel. Restoring their memories by causing their friends to remember them despite Asriel's hold on them, the player character called out to Asriel himself, who attempted to silence him before his emotions could catch up to him. Asriel broke down, explaining that he wanted to reset because of all the loneliness he had endured and he didn't want to say goodbye to his best friend. Reverting to his childlike appearance, Asriel explained that he'd been calling the human by his old adoptive sibling's name due to projecting him onto the player character, on account of the similar appearance and his own delusion. Asking for the player character's name, he confessed that he had been projecting his own kindness onto his sibling, who had been a very angry child with a hatred for humanity that he would not talk about and had fallen down the mountain with dark intentions from the very beginning, and that he didn't expect forgiveness for his actions whatsoever. He used all of his power to break the barrier separating the underground from the surface, and then returned all the souls to their proper hosts; however, this evidently came at the cost of Asriel losing the form he had created for himself, as he warned that soon he would revert to being Flowey again. He returned the player character to his friends.

As the player character walks around and talks to all their friends, however, it's possible to backtrack all the way to the ruins, and the field of golden flowers that breaks the player character's fall. Asriel is there tending them, still in his childlike appearance. He says it would be better if his parents didn't know he existed, and that "someone has to take care of these flowers;" badgering him any further results in him explaining that the First Human was significantly crueler than he or anyone else implied previously, explaining his vicious outlook on life. If he is pressed further he asks if the player character has nothing better to do. It seems impossible to save Asriel from returning to his soulless form of Flowey.

After the player exits the caverns through the opened barrier, the credits run but the screen fills with static just as the credits appear to be ending; Asriel appears in his adult form before turning to his form as a child. He tells the player to be ready for the last part of the credits, the special thanks, to be given. He does not appear if the player replays the credits sequence.

If the player reopens the game after the ending, Asriel returns as Flowey, begging the player (calling them by the name given to the First Human) not to reset, and to allow Frisk and the monsters to live happily.

Genocide Route
Even more than on Pacifist Route, Asriel - as Flowey - seems to mistake the player character as the First Human, opening about his feelings and his experiences as a flower, his incapability to feel compassion (despite spending a significant amount of time with his both parents), and how the player is the only one that understands him.

He explains how he woke up suddenly without arms, or legs as a flower, and asked for help until his parents found him. He stayed with them for a long time, but felt no compassion or comfort from their care. Eventually he ran away and tried to kill himself. However, when he came within inches of death, an incredible sense of determination overcame him, and he found that he had to ability to reset everything, back to when he was first born. Using this SAVE feature he began to play god with the world, making everyone happy, killing everyone, etc. until he became bored with the world as a whole. He reinforces his belief of the world's kill-or-be-killed nature, and how his existence has only reaffirmed his belief in a cruel world. It is only at this point, as the player is approaching Sans and Asgore that he realizes that this in no way stops the player character from killing him, too. He then changes his mind and begins to fear the player, even going to Asgore in desperation to warn him of the coming threat, to no avail.

After the protagonist defeats Sans, they approach Asgore, who they kills with ease, and Flowey, in one final attempt to save his own life, destroys his father's soul and pledges himself to the player one final time, revealing his true identity and face in the process - the one time Asriel truly makes an appearance on this route, as himself instead of as Flowey. His claims of being useful change into tearfully begging the player not to kill him, only to be cut down and utterly destroyed.

Trivia

 * Asriel is a biblical name that translates to "help of God" or "vow of God". In the story itself, though, his name is simply Asgore and Toriel 's mashed together, alluding to Asgore's inadequacy at choosing names.
 * Interestingly, Asriel Dreemurr is an anagram of Serial Murderer.
 * In addition, when pronounced, "Dreemurr" sounds like "dreamer".
 * Asriel is possibly a reference to the Jewish/Sikh/Islamic Archangel of Death, Azrael.
 * In the first half of his boss fight, the screen and his moves are colorful, as well as taking the form of how he imagines himself as an adult and then taking a form reminiscent of the "final forms" of various video-game bosses. These combined with naming his attacks (ie Chaos Buster and Chaos Sabre) are probably the result of Asriel manifesting his power in ways he considers to be cool.
 * Asriel's winged form resembles the Delta Rune.
 * The Delta Rune represents an angel who is prophesied to bring freedom to the monsters, which Asriel does in the Pacifist Route.
 * In the Pacifist Route post-game walkaround, Gerson mentions that Boss Monsters (which Toriel and Asgore both are) age by transferring a bit of their Soul to their (genetic) children, but since Asriel is dead, they can't ever grow older. However, Asriel does grow up temporarily in the pacifist ending.
 * If you type "Asriel" for the character's name, the response becomes "..." and prevents you from using that name.
 * Asriel's name is a portmanteau of both "Asgore" and "Toriel."