Michael was the firstborn child of William and Eleanor Afton. After a few years, he was given a sister, and his father founded a family restaurant. In that same year, his younger brother was born and he was dragged off to the US with his father for business. 

Things were pretty rushed for Michael at a young age, and his father was slightly neglectful. After they moved back to the UK, it got much worse when his mother left, turning into abuse. He also coped with his pain by scaring his younger brother often. But after losing both his sister and his brother to accidents, he was forcefully given eternal life, forced to leave his dad because he didn't want to live with a monster.

After moving to live with Henry, he received a letter from his father, which he followed to find his younger sister's soul. In finding his younger sister's soul, he gets his innards scooped out and replaced with a sentient animatronic amalgam named Ennard. After about a week, Ennard leaves his body, but Michael stays alive.

He decided to retrace his father's footsteps and discover where he went, which was useless. 30 years after, he decides to try again, eventually finding his father in a now dilapidated animatronic suit. Once that location burns down, he finds a new location to work at, and Henry happens to be in charge of that location. Henry trapped all the animatronics inside, along with Michael. Instead of struggling to leave, he sits down on the floor and finally gets rest after years of trauma.

***

Michael Afton was an interesting character to write. Besides Henry, he's given the least amount of personality or motive in the games. So his character was built mostly from scratch. In FNAF: Insanity, he wasn't a prevalent character because as William ignored him, so does the story. He does get the spotlight in FNAF: Redemption.

The second book wasn't just a redemption arc for everyone, it was one for Michael specifically. He got a chance to redeem his father's legacy, then got rest after years of suffering and turmoil.

For those of you wondering about his whereabouts in the 30 years, it was just him trying to live a normal life. Meeting normal people. Trying to grow old, even if he was immortal.