Flowey

Upon reaching the New Home area nearing the endgame of a Neutral or Pacifist run, random encounters in which the enemies tell the player about Asriel and Chara's history occur. When the player reaches the True Lab area of the game (which can only be accessed in a "pacifist" run in which the player spares every monster and boss that they come across) they learn the origins of Flowey—that he was the only biological son of Toriel and Asgore, being named Asriel, a combination of both of their names.[2] An only child, Asriel became best friends with Chara, the first fallen human. Chara devised a plan to attempt suicide by eating poisonous flowers, combining their soul with Asriel's and breaking through the barrier that kept monsters trapped in the Underground, rendering them free to reach the surface. Upon the execution of this plan, Asriel transformed into a powerful, monstrous being due to absorbing Chara's soul from their corpse. As the control of the body was shared between Chara and Asriel, Chara took their own body to the human village and laid it upon a bed of flowers in its center. Unfortunately, the fusion of the two was attacked by the humans who believed Asriel had killed Chara, upon seeing their body. Chara tried to fight back but Asriel resisted them and walked away. The humans mortally wounded Asriel and Chara, upon returning home, he collapsed, turning into dust, which scattered over a bed of golden flowers in Asgore's throne room. Asriel's consciousness was restored as a sentient flower as a result of scientific experimentation from the royal scientist Alphys, who injected a flower with "determination" as part of a larger series of experiments on the nature of the substance, with unbeknownst to her, the flower still having some amount of Asriel's dust on it. In the "no mercy" (popularly called "Genocide") route of the game (which involves killing every enemy across all of the areas as well as all of the bosses), upon reaching New Home, in a similar fashion to Asriel's and Chara's story in the Neutral run, Flowey tells the story of how he awakened as a flower and reunited with his parents, before realising that he no longer had the ability to feel love or attachment, due to no longer possessing a soul. He attempted suicide, after which he realised that he now had the ability to "SAVE" and "RESET" time due to the "determination" he had been given.

At the end of the True Pacifist route, Flowey retakes the souls of the fallen humans and absorbs the souls of all the monsters of the underground to take the form of an all-powerful older Asriel. Asriel, mistakenly thinking the playable character Frisk is Chara, wants them to keep playing with him over and over, fearing losing them again. Frisk "SAVES" Asriel and reaches out to him, leading the monster to emotionally break down and admit his insecurity and loneliness. Asriel regains his empathy and reverts to his original appearance as a child. He apologizes for his misdeeds and uses his power to break the barrier, thus freeing the monsters. He ultimately reverts to Flowey due to releasing the souls of the humans and monsters, once again losing the ability to feel love. In game, it's ambiguous as to whether Flowey leaves the Underground with the others, but fan speculation and certain supplemental media suggests that he does.[3]

Towards the end of the "no mercy" route of the game, Flowey flees to warn Asgore of the player's cruelty. The player encounters Flowey pleading to Asgore in his throne room, and without player input, the playable character mortally wounds Asgore. Flowey finishes off Asgore to prove his loyalty to the player, but realises that he may be the player's final target, and begs for mercy, saying that he can be helpful and will not make a mistake again, revealing himself to be Asriel by mimicking his former face and voice. He is ultimately hacked to pieces, again without any player input, after which Chara appears to the player and, after a monologue about the player's destructive actions in which they mention how the player's actions made them realise that their purpose in life is power, destroys the world.

In Deltarune, which takes place in a different universe than Undertale, albeit featuring many characters who first appeared in that game, Asriel appears as the older brother of Kris, the main protagonist, and the first son of Toriel and Asgore, and is said to be away attending college. When Kris and their classmate Susie enter the Dark World, they meet Ralsei, a prince from the dark whose appearance and personality are similar to that of Asriel. At the end of the first chapter, before Kris and Susie leave the Dark World, Ralsei reveals his true appearance, previously cloaked in shadow under his wide-brimmed pointy hat, which heavily resembles Asriel's appearance in Undertale. Additionally, Ralsei's name is an anagram of Asriel.