While Mike Schmidt is the protagonist in both the first “FNaF” game and the movie, his backstory is completely different. In the games, Mike is William Afton’s son, and “Mike Schmidt” is just an alias he uses to earn the security guard position at Freddy Fazbear’s Pizza to put an end to his father’s crimes. In the film, he takes the job as a last-ditch effort to show employment in order to maintain custody of his little sister, Abby.
Additionally, Abby is not a character in the games. There are a lot of little girls in the game’s lore (Elizabeth Afton and Charlotte Emily, most prominently) but not Abby. This change is a positive one because the eventual reveal that Mike was William’s son always had a bit of a Darth Vader/Luke Skywalker vibe to it all and felt a bit silly. By turning Mike into a character who wants to fight Afton because it’s the right thing to do rather than one motivated by a familial obligation, his quest for heroism feels stronger.
Instead, Mike’s true motivation in the film is focused on his family. He’s still reeling with the guilt of his brother Garrett being kidnapped (by William Afton), and trying to keep Abby safe. Afton is still very much a threat to his family, the way he is in the games, but Mike no longer has a familial bond with the killer. This offers room to expand Afton’s character to something brand new, something that even die-hard fans can’t predict every step of the way. It also allows the Vanessa character more room to grow.