W. D. Gaster (Fanon)/Withersoul 235

"It hurts me so much that no one ever cared about my disappearance. Amnesia or not, this was unacceptable. Why am I here, you might ask? That Alphys... warn her that her life is coming to its conclusion. But wait! You cannot. Because I already broke her path of life. You should feel how it is to be forgotten by everyone. You should feel how it is to see a world where you no longer exists, yet everything goes on perfectly without you. As if you weren't ever needed. As if no one ever asked for your existence. As if no one gave a damn. As if no one ever cared. That's what you'll feel. Alongside pain... the pain of mental and emotional trauma. The pain of depression. The pain of uncontrollable bleeding. The pain of no longer being a thing. The pain of pains. The eternal pain that will scar you for life."
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- W. D. Gaster

"Tra la la. Beware of the man who speaks in hands."

- Gaster referring to himself in his River Person form

Summary
Walter Douglas Gaster, better known as W. D. Gaster, Wing Ding Gaster or simply Gaster and referred to as the River Person in his hooded disguise is the main antagonist of Uppertale. The former royal scientist of King Asgore Dreemurr, Gaster was admired all over the Underground in his time as trustee of the monster government - except by his assistent, who was jealous of all attention he got. One day, months after the deaths of Asriel Dreemurr and the Six Human Souls and shortly before the new royal scientist Alphys would create Flowey, Gaster invented a machine that could warp space and time as instructed by Asgore in an attempt to warp the monsters beyond the barrier without requiring any more souls. Gaster would stay behind and then flip them back after the town that murdered Asriel was burnt down.

But this didn't go as planned. Gaster's jealous assistant found this the perfect opportunity to dispose of his master and take all credit for himself. When Gaster was testing the machine, the assistant pushed him in, causing his essence to be shattered across nonexistence which led to all traces of Gaster's life being deleted, safe for the memories of the assistant, Gaster's followers and a few others. Minutes after, the assistant mysteriously disappeared - unbeknownst to anyone, Gaster's influence took the assistant down with him and this time erased all traces except for Gaster's memory of him. The few that remembered Gaster concluded that it was an incident due to one of his inventions having failed.

Aside from a few hidden and cryptic appearances, Gaster's appearance in Undertale were limited - until it was revealed that the very River Person who ferried Frisk through the Underground is in fact Gaster. In Uppertale, Gaster appears as the main antagonist of the game, bent on eliminating Alphys. This soon advances into a desire to punish monsterkind for their supposed 'misdeeds', then into corruption and finally into a desire to use his tech to rule the world.

After the monsters reconciled with humanity and finally came to live together and in harmony and peace once more, Gaster made his first move from beyond the grave upon the resurrection of Chara and Asriel and their return to the Underground to feel the nostalgia. Ambushing them when Toriel and Asgore weren't around, Gaster took control of their thoughtwaves and mindset, brainwashing them to become his servants. Soon, more and more of Frisk's friends followed. Upon finally joining the newly re-founded Royal Guard, Papyrus was tasked with hunting Gaster down after Undyne was hypnotized and kidnapped, unaware - just like everyone else - that Gaster was the very person that ferried him and his troops around. Taking on various shapes, possessing various lifeforms and manifesting himself within several objects, bionic lifeforms, locations and concepts, Gaster freely manipulated the entire cast and story in a giant gambit without being suspected. But then, Papyrus finally found out the truth after the missing Sans was found, who, unlike the other victims of Gaster, didn't suffer amnesia, and told them the truth about the enemy having infiltrated within their ranks, having been with them the entire time. The River Person was confronted in the throne room of Asgore's surface residence. With the cast still unaware of the River Person's true colours, Gaster took the opportunity to warp the fabric of reality.

Capturing Frisk's friends once again, the River Person iniated a short fray with the child until forced to reveal his true identity as W. D. Gaster. While corrupting existence and fusing all worlds and subspace into one big super realm, Gaster forced Frisk into killing all of his friends, swapping places with them through reality warping to use them as living shields, controlling them like puppets to force them into attacking Frisk and finally absorbing their SOUL, life-force, chi, aura, chakra, nen and essence to achieve an immense level of power that made him almost completely omnipotent without limitations and become a transcendental, metaphysical god that rules over creation.

But once loaded with suffient loads of DETERMINATION, Frisk finally managed to break through Gaster's defences after being plunged through various other battles - including fights with clones of the previous game's characters - and finally engaged into the true final fight. Halfway through, Gaster tries to manipulate Frisk into sparing him - backstabbing and killing him if he does so - but the child doesn't fall for the scientist's tricks. After more fighting, Gaster offers to spare Frisk, resurrect his friends, and leave them be, even destroying all buttons except for the "SPARE" button to prove that he can be trusted.

Frisk agrees to end the fight peacefully, but Gaster backstabs him again and one-shots Frisk with a giant burst of energy and proceeds to absorb him fully to achieve omnipotence, but right then, the Annoying Dog steps in, resurrects Frisk and all of his friends and shatters Gaster's defences. After a few more hits, Gaster is seemingly finally done for, but he soon charges up a giant hypernova attack to undo creation. However, all of Frisk's friends bestow him with their SOUL energy to give him the power to finally end the scientist. With one final attack, Gaster is finally crippled and is left with only the upper half of his body. In a final attempt to kill his adversaries, Gaster proceeds to overload himself with such tremendous amounts of energy, it would cause him to implode and blow up the entire area to kill Frisk and co, but they manage to escape just before the detonation, leaving Gaster to only destroy himself and accidentally commit suicide in the process, finally ending the crazed scientist once and for all.

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