Flowey the Flower is the deuteragonist and Frisk's partner in Underfell's true pacifist timeline. He is also the heroic antagonist in the Genocide timeline.
Before he became Flowey, he was Asriel, King Asgore and Toriel's son. After his adoptive sibling Chara died of Buttercup poisoning, Asriel used Chara's soul to cross the barrier. He had transformed into the "Hyper Angel of Life." He granted Chara's wish to die peacefully by lying them in a bed of golden flowers. Unfortunately, the humans mistakenly thought he killed Chara and approached him, armed to the teeth; however Asriel refused to fight and showed mercy, which they did not accept. They proceeded to brutalize and maim him, leaving him for dead. Still barely breathing, Asriel returned to the safety of the Underground -- before promptly dying from his fatal injuries. His dust blew onto a golden flowerbed, where he would be soon to wake up, reincarnated as a living golden flower. Afraid of what his parents might think, he ran away.
The king was so devastated that he began to fester a hatred for humanity, wanting them dead for their crimes against his son. This grudge became so strong that he eventually declared war on the humans with the intent to punish them for the villagers' mistakes. Toriel, disgusted with his actions, left Asgore's side. In his rage the King declared a new law: that it was "kill or be killed", and he had no intentions of revoking it.
Flowey, like his UnderTale counterpart, still believes in the "Kill or be Killed" motto, afraid of other monsters and desperate to do what it takes to survive. However, after meeting Frisk, he starts to believe in a different motto: "Live and let Live."
UF!Flowey is relatively similar to his UT counterpart in terms of appearance; however, Underfell Flowey's petals are visibly tattered and shredded, similar to how Flowey appears at the end of the Omega Flowey fight in the UnderTale universe. His eyes are small, beady and black. His face is a soft white and his petals are yellow.
Flowey is shown to be similar to UnderTale Asriel - usually happy, but can also get upset and cry a lot. He is afraid of the monsters in the Underground and is terrified of King Asgore, despite Asgore being his father. He is the opposite to UnderTale Flowey- where while UnderTale Flowey wants to hurt you, Underfell Flowey doesn't.
Powers and Abilities
Flowey has the same powers as his UnderTale counterpart, but uses them differently. He can manipulate "friendliness pellets" and tactfully weaponize them -- though he only ever uses them in self defense. He can also morph his facial expressions to an often scary extent -- but, again, will usually only do so to defend himself and scare off any threats. This normally works out for him.
Alpha Flowey - The fight against Flowey takes place in the Genocide run. Flowey realizes that you've become an even worse monster than the monsters underground, and decides to try and stop you.
Asriel Dreemurr, the Hyper Angel of Life - After losing to you as Alpha Flowey, Flowey is about to give up, but realizes that if he gives up now, humanity and the rest of the monsters WILL perish. Knowing that the fate of the whole world is on his "shoulders" now, Flowey generates enough determination to transform to "Asriel Dreemurr, the Hyper Angel of Life".
Frisk - Flowey accompanies Frisk on their journey to escape the Underground, as well as avoiding monsters together. Flowey offers to protects Frisk from the dangers of the Underground despite the fact that he is terrified of the monsters that are around of the Underground. He will occasionally project Chara onto Frisk, but usually only in moments of vulnerability.
Sans - Sans and Flowey hate each other.
Toriel - Flowey knows about Toriel and her insanity, even trying to warn Frisk about her. He is afraid of Toriel, even though she's his mother.
Asgore - Flowey is terrified of his own father.
Chara - Before Flowey became a flower, he was Asriel Dreemurr. He saved Chara, and this is where the Dreemurrs adopted them. Chara was very close like in UnderTale. She and Asriel shared a much stronger relationship than in UnderTale, though, because Chara didn't want to destroy humanity but get away from it, so Asriel saw Chara as special. He even sometimes accidentally calls Frisk "Chara."
Most parts of this article is taken from Underfell Wikia Credits to the original Authors