Also, Frisk is an orphan. That one was pretty easy, as their parents never bother to look for them.

Sans, Papyrus, and Gaster worked together for a long time until Gaster fell into his machine. This drove Papyrus to repressing his memories and maintaining his work ethic, whilst trying to join the Guard for unexplained reasons (the guard is close to the king and the king might be able to find someone to bring back Gaster). Meanwhile, Sans fell into crippling depression, and in his anger and sadness developed his powers. However, in order to be more accepted, he hides it and puts up a front as just some guy who likes jokes. In a Genocide run, once you kill Papyrus, Sans is furious as he has now lost both of his siblings to the humans (the device was to prepare for the surface), and takes personal revenge. Once you kill him, he experiences a brief moment of bliss as he can now be with his brother. Mettaton finds him and brings him to Alphys, as he does with Papyrus, and Alphys does her best to heal them. However, the best she can do is to fuse them together to prevent further harm, creating the Bromalgam. Once Chara kills the Bromalgam, they/it feels happy instead of pained at death.

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