Flowey

"In this world, it's kill or BE killed."

- Flowey's catchphrase. "Howdy! It's me, Flowey. Flowey the Flower! I owe you a huge thanks. You really did a number on that old fool. Without you, I never could have gotten past him. But now, with your help... He's DEAD. And I've got the human souls! Boy! I've been empty for so long... It feels great to have a soul inside me again. Mmmm, I can feel them wriggling... Awww, you're feeling left out, aren't you? Well, that's just perfect. After all, I only have six souls. I still need one more... Before i become GOD. And then, with my newfound powers... Monsters. Humans. Everyone. I'll show them all the REAL meaning of this world."

- Flowey the Flower

Flowey (originally known as Asriel Dreemurr) is the true main antagonist of the 2015 videogame Undertale, and the first character the protagonist meets after falling into the Underground. He appears as a small golden flower.

Past
Originally known as Asriel Dreemurr, he was born the child of Asgore and Toriel, the king and queen of the monsters. After a war against the humans forced all monsters underground, sealed away with a magical barrier, he lived his childhood separated from humans; however, when a child fell through the barrier, his family adopted him as their own. He soon became fast friends with the human child, and the two of them would do anything together.

Some time later, the child became deathly ill, and told Asriel of their desire to be returned to the surface and buried in his home village. Taking his sibling's soul, Asriel was able to breach the barrier and enter the human world, but when seen by the humans, he was accused of killing the child and was subsequently attacked. Despite having immense power from both a human and a monster's soul, he refused to fight back, and managed to return to the Underground before finally dying over a field of flowers. (It later became clear that the child deliberately poisoned himself, and wished to use their combined powers to kill the people in the village.)

The royal scientist of the monsters, Alphys, was tasked with finding out a method of breaking the barrier without the use of a human's soul, and in doing so discovered the existence of a substance known as Determination, which kept human souls powerful even after death. After tests on giving Determination to monsters proved unsuccessful, she decided to test the substance on an object without a soul, one of the flowers Asriel died over. As a result, he was reborn within the flower, with all his old memories, but without his soul.

Asriel attempted to live life with his father, and then with his separated mother, trying to regain his soul and emotions, but was ultimately unsuccessful. In the end, he ran away and attempted to kill himself, but when he changed his mind at the last minute, he found himself returned to an earlier point in time. He discovered that his Determination allowed him to undo and reset time itself, like saving and reloading a video game, and began to experiment with his new power. At first attempting to befriend all the monsters around him, he soon grew bored with being able to predict people's reactions, and turned to violence instead. Now calling himself Flowey the flower, he would kill every monster in the underground he could find, reset time, and repeat the process over again.

After a certain point, however, he found he could no longer alter time. The cause, he discovered, was due to a new child falling into the Underground, someone who looked just like his sibling from the past with powers of Determination able to override his own. Flowey then decided to use this child to his own advantage, leading them down a route where they would either die and return the power to him, or allow him to attain even greater power.

Undertale
Flowey introduces himself to the player at the start of the game, pretending to be a friendly figure introducing the mechanics of the game, but quickly reveals his true nature and tries to kill them immediately, only to be stopped by Toriel. After the child leaves the ruins and travels further into the Underground, he taunts the player based on whether or not they had killed anyone, before disappearing again. He follows the player through the game, staying just out of sight.

Pacifist Run
This ending can only be achieved if the Neutral Run has been completed previously.

In the True Pacifist ending, Flowey organizes for every monster in the Underground to gather in Asgore's castle, and uses the opportunity to steal all their souls at once. With so many combined souls, Flowey is able to finally take his true form as Asriel Dreemurr, and challenges the player head-on for the return of their souls and a "happy ending". As they battle, he expresses his true desire, to be able to play with his brother for all eternity, no matter who is sacrificed in the process. He is ultimately defeated when the child reaches and frees the souls of the monsters trapped inside him, and finally frees his own soul. In a moment of pure lucidity, unaffected by emotions of rage and guilt, Asriel finally reaches peace with the child; he acknowledges that his sibling was not a good person, and releases the souls he had taken, after using his power to destroy the barrier and let monsters return to the surface.

In the epilogue, if the player goes back to the first area of the game, they'll find Asriel there, saying that he's decided to stay underground, so that he will not harm anyone after he turns into Flowey again. If the player starts up the game after the True Pacifist ending, Flowey will appear to try to convince the player not to reset the game and everyone's happy ending, before saying that the player's must have already heard him say it a hundred times and leaves.

Neutral Run
In the Neutral ending, Flowey appears just before the player can kill Asgore, and deals the finishing blow himself, destroying both his own father and his soul. He then steals for himself the six human souls that Asgore had collected in hopes of destroying the barrier, and transforms into a horrific monster known as "Omega Flowey" with full control over time. Out of sheer pleasure, he traps the child in an endless battle, always bringing them to death or near death and then resetting time to do it all over again, while at the same time saving and loading moments in time when he can best hurt them. However, when the child calls out to the six human souls inside of him, they rebel against him and seal away his godlike powers. Completely helpless, he is left to the player's choice to either be killed or spared.

If killed, Flowey will acknowledge the player for killing him and turns into just a plain flower. he doesn't appear at the beginning if the player restarts, but still remains at the end. If spared, Flowey will get confused as to why you've decided to spare him and gives you several opportunities to kill him. If you keep sparing him, Flowey will state that he does not understand before running away.

Genocide Run
In the Genocide ending, Flowey is downgraded to secondary antagonist, the true main antagonist being Chara possessing Frisk. If the player keeps murdering all the monsters on his way, Flowey finally realizes that the child was growing more evil than himself, and that he was in imminent danger of being killed. He quickly escapes to warn Asgore of the coming danger, while the child is distracted fighting a final wave of defense. When Asgore is ultimately brought to near death, Flowey reappears and deals the finishing blow, claiming to have been on the child's side all along crying and begging for forgiveness; thus, revealing his true identity as Asriel Dreemurr. However, his pleas are ignored and he is mercilessly killed by the Fallen Child (Chara).

Trivia

 * Flowey acts as the true main antagonist at the end in place of the lead-up main antagonist, Asgore, due to the latter being revealed as a kind and sorrowful character.
 * If one backtracks a bit after certain points, they can catch a quick look at Flowey for a split second before he retreats underground, showing that he's been stalking you, even up to the end of the game.
 * Flowey is similar to Giygas from (Mother) as both in their final forms are unrecognizable abominations, are almost invincible and are willing to kill the protagonist at any costs. In addition, at one point the protagonist calls for help to defeat them.
 * Oddly enough, Flowey is also similar to Porky Minch from the same franchise, as both apparently acted as minor antagonists at first but then they revealed themselves as major antagonists. Not to mention that both taunted the protagonist using the same insult during the final boss battle.
 * Flowey is very similar to Dimentio from Super Paper Mario.
 * They both initially appear to be minor antagonists.
 * They both kill the apparent main antagonists after the (seemingly) final battle, right after the fake main antagonists reveal their tragic motives.
 * They even both kill them using their trademark attacks.
 * They both end up becoming the true, much more disturbing and godlike final bosses.
 * Flowey is also similar to Darth Vader as:
 * Both used to be on the good side, acting kind and nice to others until both were led to the dark side by a corrupting force (Chara for Asriel, Palpatine for Vader).
 * Both had to leave their true selves after turning to villains, Asriel to Flowey and Anakin to Vader.
 * Both hold a sort of care to the protagonists even if they already knew they were enemies at first.
 * Both at the end are redeemed and die with honor.
 * Both are also biologically related to one of the protagonists (Flowey/Asriel as Toriel and Asgore's son and Darth Vader as Luke's father.)