Flowey is a demon that originally lives in Hell.
History
Flowey was a demon from Hell, but he ran away, killing many demons. Hidden in the Underground, he hid his true self inside a flower. Then, he waited and waited to trick a human into killing every monster in the Underground.
Flowey first killed Toriel just to irritate Chara. But Chara brutally killed him with a kitchen knife. Luckily, his DETERMINATION allowed him to persist after death, and be revived.
Flowey attempted to kill the 7th human, Frisk, but failed to do so, and gotten hurt by Toriel. Flowey kept greeting Frisk, and tried to turn Frisk into what he'd done to Chara, which made her a murderer.
Although, Frisk resisted his trickery. She kept sparing the monsters left by Chara and eventually, left the Underground and to the Surface.
Appearance
Flowey appears to be a daisy, but with a face.
Personality
Like Chara, he has no feelings of love, but covers his weakness by acting caring to others. Really, he doesn't like anybody and wants to kill them all.
Attacks
Flowey attacks with pellets, that are actually bullets that are stronger than titanium.