Flowey

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

Flowey is the main antagonist of the 2015 video game 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 Dreemurr, 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.

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 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.

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. He then decides to stay underground, so that he will not harm anyone after he turns into Flowey again.

In the genocide ending, Flowey 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 and begging for forgiveness; however, his pleas are ignored and he is mercilessly killed.