Gaster

Gaster is an unlockable skin in Bonetale.

In the old mobile version of the game you can unlock Gaster by playing as either Undyne or Papyrus for the first time. Frisk will then show up as Gaster with a text box full of glitch text, before the game crashes. When you reload the game, you will have the opportunity to purchase the Gaster skin.

In the PC and new mobile version you need to buy the "True Lab" map and there is a chance that you will meet him.

Having your skin id zero (must edit game files), by default, you will get the gaster skin. This is not obtainable any other way, and he has the same sprite as gaster himself.