Flowey is a demon that originally lives in Hell.
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 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.
Flowey appears to be a daisy, but with a face.
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.
Flowey attacks with pellets, that are actually bullets that are stronger than titanium.