Empty!Frisk is an out!Code created by Someone Said This. They are a Soul Human and the companion of Void!Chara. They came from the AU, Undervoid.
After Frisk and the other monsters escaped the underground, Frisk got very ill. They went to the CORE to meet Flowey. but as they were about to leave, the illness kicked in and the fainted, falling into the core. but when CLASSIC!Chara saw this, they gave their soul to make sure that Frisk didn't end up like r e t s a G. They then had a soul, but no body, and had to possess someone before they shattered. But they couldnt bring themselves to take someones life to restore theirs. but then they found out about the VOID experiments and figured out that VOIDHumans and SOULHUMANS (yes this has happened before, it's a species) could coexist. and so they fused with the last VOIDHUMAN's SOUL, and they coexist, becoming best friends.
the Voidhuman that they fused with.
- Their AU have a comic, which can be found here.