Here's theory about them: in video game series. It revealed that Mike Smith and Michael afton believed to be one and same. This is because Mike Smith was Michael afton himself who did Renamed himself to it and that was true

However, in movie/films, unlike their original/video game counterpart, Mike Smith and Michael afton are separate character in this fnaf movie as well

A difference, Michael afton in video game (fnaf series) was hero/good characters and not villain while Michael afton in movie was villain who's William afton's son as well as Vanessa's older brother. He was controller of toy animatronic and serial killer like his father

But at end of films, he's still alive and not died yet because he did escape from it after original animatronic destroyed toy animatronic