In Chapter 6 of Celeste, there is a hidden developer room containing various characters. Presumably these characters are from other games by Celeste's developer, Matt Makes Games.
To find the room:
In Chapter 6, go to the Reflections checkpoint to start by a broken elevator. Air dash upward and climb the wall to the next screen. Next, jump and dash up to do a boosted wall-kick off the first small ledge on the left to reach and grab second ledge. Jump towards the right side to find an invisible alcove to recharge your dash. Finally, do a dash-jump across the pit, then dash upwards into another small invisible passage. (Video demonstration)
Who are the characters in this developer room? What games are they from?