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Before the events of Undertale, Asriel found the fallen human, who was the first human to fall into the Underground. They became his adoptive sibling and best friend. Just before the human died, their last wish was to see the Golden Flowers of their village, on the Surface. Asriel absorbed their SOUL, and the human carried their own body across the barrier to their home village. Asriel and his sibling shared control of Asriel's body after their SOULs had combined.
The villagers assumed that the Asriel/Human fusion had attacked and killed the human child. The Asriel/Human fusion did not fight back but only fled. Lethally wounded, the Asriel/Human returned to the Underground and died. Once deceased, Asriel's body turned to dust, which spread across the garden in New Home.
Later, during the determination experiments, Alphys injected determination into one of the garden's Golden Flowers — the first one to bloom after Asriel's death. Initial experiments on the flower vessel proved unsuccessful, and Alphys returned it to Asgore. Not long after being replanted, Asriel awoke within the garden, reincarnated as Flowey.
Though the injection of determination granted Flowey the will to live, he soon found that neither his mother nor father could elicit any emotional response from him. He did not have a SOUL and thus could not feel love, joy, or compassion. He begins to reset an countless amount of times doing everything the world has to offer before eventually meeting Frisk.
Flowey introduces the concept of the Bullet Board and calls his bullets "friendliness pellets."
If the protagonist dodges three times, his friendly act slips and he hypocritically snaps at the protagonist for leeching off of him.
The background music lowers by one semitone each time the bullets are dodged, before slowing, lowering the pitch, and stopping entirely.
Before his third attack, Flowey questions the protagonist's mental capacity, before bellowing in frustration "RUN. INTO. THE. BULLETS!!!" before quickly looking directly at his text and changing the "BULLETS!!!" in the speech bubble to "friendliness pellets."
If the protagonist runs into his bullets, he announces his central philosophy. Regardless of whether the protagonist dodged Flowey's bullets or not, he then attempts to kill the protagonist with an unavoidable ring of bullets, but Toriel intervenes and saves the protagonist.
If the protagonist killed Flowey on a previous Neutral Route (or if they aborted the Genocide Route, even if they spared him), he does not interact with them again until the protagonist reaches New Home again. If the protagonist spared him, he appears in the beginning of the Ruins again and advises them on what to do. If the protagonist gained EXP, he reminds them not to kill anyone. If the protagonist gained no EXP, he reminds them to befriend everyone. Resetting the game before completing a Neutral Route causes him to repeat his "tutorial" after assuming that the protagonist is pretending not to know who he is. Resetting again causes Flowey to become annoyed and question if the protagonist has anything better to do.
If the protagonist resets a Genocide Route at any point after exiting the Ruins and before the route's completion, Flowey says "Really, <Name>? Well, do what you will. I'll be waiting for you!" before leaving.
At the exit to the Ruins, Flowey judges the protagonist's actions up to that point. Flowey does not appear to judge the protagonist if he was killed in a previous Neutral Route or if the protagonist aborted the Genocide Route in a previous playthrough.
After the Ruins, Flowey is seemingly absent until the end of Asgore's battle. However, backtracking in some rooms allows the protagonist see Flowey burrowing back into the ground on the edge of the screen.
Flowey also leaves a message on an Echo Flower in the vertical room before the bridge in Waterfall that changes if the protagonist spared or killed Toriel.
If the protagonist decides to kill Asgore, Flowey appears, destroys Asgore's SOUL. If the protagonist spares Asgore, Flowey appears and kills Asgore himself. Either way, he absorbs the six bottled human SOULs and transforms into Photoshop Flowey. Then, the game crashes and Flowey's battle begins after opening the game again.
After he is defeated, the protagonist has the chance to spare or kill Flowey. Flowey requires the protagonist to offer him mercy multiple times before he flees, unsure why the protagonist is being nice to him.
If spared after his battle, Flowey gives advice to the protagonist on what to do during subsequent Neutral Routes. If the protagonist gained EXP before his battle, he asks the protagonist to prove his philosophy wrong, as he thinks it may be meaningless. Flowey also offers unique dialogue if the protagonist only killed Asgore and tells the protagonist that they wasted everyone's time. If the protagonist earned no EXP, yet they have not met all the requirements for the True Pacifist Route, he mentions that the protagonist should get closer to one of the main characters, depending on who they previously befriended.
The protagonist can then recover their previous SAVE file and follow his advice.
If the protagonist kills Flowey, he recognizes that the protagonist can kill before becoming "just a regular flower."
If he was killed after his battle, Flowey does not leave a message for the protagonist after the credits close, and, after a reset, does not appear until after the resolution of Asgore's battle. Flowey then berates the protagonist for thinking that killing him solved anything. He explains that he was brought back when the protagonist loaded their SAVE file and that they still cannot achieve their happy ending.
If the protagonist aborts the Genocide Route, Flowey becomes upset with the protagonist. However, he then proclaims that the protagonist just wanted to see everything in the Underground and will eventually return to their malicious ways.
If the protagonist loads or finishes another Neutral Route, Flowey does not absorb the SOULs because he knows that they would rebel again. Instead, the protagonist is able to exit the room and receive Sans's Neutral Route phone call.
In the Genocide Route, after the protagonist kills Toriel, Flowey tells them that they are not really human, and mistakes them for the first child, telling them that they are still inseparable after all those years.
When the protagonist reaches New Home, Flowey welcomes them and discusses his backstory as a flower and how he discovered his ability to SAVE. Near the end of his dialogue, he realizes that the protagonist will not hesitate to kill him, and he is frightened for his life. He rescinds his previous plans with the protagonist and tells them to leave him alone.
After the protagonist defeats Sans, they easily kill Asgore. Flowey destroys Asgore's SOUL and pledges himself to the protagonist one final time in an attempt to save his own life, revealing his true identity as Asriel in the process. He cries and begs the protagonist not to kill him and the player has the opportunity to spare him by quitting the game or deliver the final blow.
During the True Pacfisit Route, in the True Lab, the protagonist discovers that the first human and Asriel had worked together on a plan to get to the Surface and retrieve six human SOULs. While the human was on their deathbed, Asriel informed them he did not think it was a good plan anymore; ultimately, this leads to him betraying his sibling and aborting their plan.
After restoring the elevator's power, the protagonist receives a phone call from someone whose voice they have never heard before; the sound bite is similar to Asriel's.
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