According to some redditors, some portions of the game have been leaked. Not saying which, I don't know what's in them. But I'm closing down my markets to prevent spoilers. Just letting you know.

why isn’t this 100% he is literally the first character to appear in the game

A few things make it pretty likely, IMO:

- The game was announced on Twitter in his voice/speaking style

- The opening narration is consistent with his voice/speaking style
- The theme that plays during Deltarune's opening narration is called "ANOTHER HIM" and is a rearrangement of "mus_st_him," which is Gaster's theme song

- The "vessel" created at the start of the game is in grayscale, while other characters are in full color, like the Goners from Undertale

Still no outright confirmation, but I'd be willing to bet a lot of mana that he will appear at some point.

@evergreenemily it will be very interesting to see how this market develops over the next 20 years (I am just assuming the worst with how long it will take for this series to be completed)

@Chizardium Very, very interesting.

@Chizardium Deltarune will 100% be finished by the end of this decade