The Man Behind the Tree in Deltarune is DEFINITELY Gaster. It's already heavily suspected that this is the case, but in Deltarune Chapter 3 or 4 (i forgot which one but this happens) Kris can try to draw the Man Behind the Tree, only to have to restart... Similar to how if you try to enter Gaster's name in Deltarune, the game restarts. Also, in order to get to the MBTT room, you have to enter and exit a certain room multiple times. This applies for each chapter. In this area in Chapter 2, the music restarts each time you go between rooms. If you enter and exit quickly, it sounds very similar to Gaster's theme.
