MainEditFlowey is a character, and primary antagonist of Uppertale.
He welcomes the player to Uppertale. He is first confused by you playing this game as you got the 'happy ending' you deserved already, at the end of Undertale. He eventually lets you go saying not to expect him to be nice to you.
You encounter Flowey after time traveling to Asgore's castle and initiates a battle with the player. If you choose to talk to him multiple times, he will eventually get frustrated, and take the human souls himself. He then crashes the game. After reopening the game, a fight with God Flowey is initiated. You can not die in this fight as your determination doesn't allow it. Eventually, beating him will restore the world to what it once was before.
During a Genocide run, you kill Flowey after killing Papyrus & Sans.