Final Sans is a God version of Sans, with almost complete power over his world. He normally chooses to be in his normal form, which is just a basic Sans, except he knows that he knows he is in a game. Neutral and True Pacifist are the same as normal Undertale. He also is much more merciful than normal Sans, as he knows that he cannot kill the human once and for all, because of their determination, so he chooses to try to change them and make them repent. Because they are in the Genocide route, he does not choose to make jokes, and his character is slightly changed, making him sarcastic at points in his battle.
Genocide:
If the player does genocide, Sans will make several references outside the fourth wall, hinting that he knows he is in a game. In addition, whenever he says "You're going to have a bad time," the text is in gold. After the player kills Flowey, instead of meeting Chara, they meet Sans again. His wound has healed, and he is glowing gold. He then talks to the player and offers them a chance to reset the game and do Pacifist. If they choose to try to attack him, he will start glowing red instead of gold, and begin his attack. At first it is just basic Sans, except he does two damage per frame instead of one. If the player survives his battle and chooses to attack him once again, his face falls and he offers them one more chance to spare. If they choose to attack him, the screen goes dark, then turns white, and his face, now three times as large and with a silver tinge, appears. He talks to them, then begins the final battle, in which he is now in his true form, Final Sans. At the beginning of the battle, he reduces the humans health by half.His attacks now do five damage per frame, and his karma is black, making it twice as powerful. He then turns the fight button into the spare button, effectively removing any chance of killing him. If the player chooses to ACT, three choices will appear. If they choose to check, it will simply say "You don't deserve to know." If they choose to run, the first time it will say "Coward!" and nothing will happen. The second time they select run, Final Sans will add the choice of resetting the game. Finally, if the player chooses to taunt, Final Sans's attack increases by two. To beat Final Sans, the player must choose to fake sparing by choosing it inside the SPARE twice, then choose run. Then the player must choose to check, and if they press Z several times during Sans's attack, he will stop attacking and say, "You LOVE pressing that attack key, don't you? Well, here's a chance for you to use it." This will cause the fight button to appear once again, and if the player chooses to fight, Final Sans will instantly kill them. Instead, they must choose to taunt, then attack. This will cause Final Sans to get angry. This will allow the player to choose strong attack in the ACT, which will knock Final Sans over, ending the battle. He is now asleep, however the player cannot kill him because of a golden force field around his body. If they choose to walk back into new home, they are presented with an option, which allows them to reset the game. If they choose this, and they do genocide route again, at Sans's boss battle, he will instantly kill them and delete the game from their computer, effectively stopping them forever.However, if they instead choose to go past Final Sans, they will find a giant, black stone blocking their path. They can then choose to either repent of their sins, which causes a dark soul to fly out of them and into the stone. They then awaken Final Sans, which immediately changes the scene to the ending of the True Pacifist route. However, they can also choose to put their hand on the stone and force it by the power of their LOVE onto Final Sans, breaking his force field, but also awakening him. However, he is trapped under the stone, and asks them if he can tell them a story. If they accept, he tells them about how he became Final Sans, and pleads with them one final time to spare him and reset the game. If the player refuses, they can then go to the barrier, and find a giant, dark presence waiting for them, which instantly kills them if they try to fight it. If they go to the stone, however, it will give them the option of trapping Final Sans in the stone forever. If they accept, Final Sans is sucked into the stone, leaving only a note that reads: "You still have the chance to reset. Remember the Pacifist route." It then gives them the chance to reset one final time. If they refuse, the screen goes black, and text appears, saying: "You killed everyone. EVERYONE. And what did they ever do to you? Welp, nice job. You completed the true genocide route. Goodbye, Fallen Human." The game then closes, and opening it again only produces a black screen.