Shadow Doggo is an easter egg character, and perhaps the most mysterious character in the game. It usually randomly appears when switching from room to room, Naming yourself Shadow Dogger or Shadow Doggo, or in the several Minigames.
Shadow Doggo is well known for saying dialogue whenever you see it, but it's always scrambled in some way. It also appears always in certain endings, mainly the "An Ending", which the encounter can change the game forever and reveal more lore. There is a instance where Shadow Doggo's speech is not scrambled and is readable, in one of the DSAF 2 Endings. If Shadow Doggo is entered as the player's name, he will appear and state that, when translated, is "'IMPOSTER'".
According to DirectDoggo, Shadow Doggo's backstory will be revealed in the third game.