Flowey

Flowey has the same generic story as every other Flowey, died as Asriel with Chara's soul and was turned into a flower because of Alphys's experimenting.

Flowey's Themes
(When you meet Flowey): Happy to Help!

(Omega Flowey pre-fight scene): You are such a fool!

(Omega Flowey's fight): Walls of Vines and Nightmares