Blackjack (or Shadow Doggo) is the soul of Jack Kennedy.
Blackjack 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 Blackjack'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'".
In Dayshift at Freddy's 3 in the Good Ending route, it is revealed that Blackjack is actually Jack's soul, which took form of his childhood dog, Sparky, and trapped Henry in the void. He is also given to Dave by Jack when he refuses to move on without him.