so about that backstory
There's only really three set dates in Fitz's life- his birth year (1932), the year he left for Rapture (1952), and the year the game takes place (1960).
He's from NYC, where his family is several generations removed from Irish immigrants. They were always pretty poor; not starving or anything, but frugal enough that he'd go a year or two with one big pair of pants growing up. I'm not sure what his father did, but he had a mother and 6-or-7-years-older sister who were always at home.
Being in the big apple, he had a lot of exposure to the arts, and he started working part-time at 11 or 12 to fund piano lessons for himself. A lot of it was with his father's support; the rest of his wages just went into family funds anyway. He was a really precocious player, and after he was a little older, he would offer to play free at bars and places, in exchange for being able to practice there after they closed.
His relationship with his parents before he left was always fine. His mother liked to dote on him after his sister was married off, and his father was supportive of ambitions beyond a factory job, especially after he realized his son was actually very talented. Fitz never did end up graduating high school, but his grades before dropping out were average at best, as was somewhat expected of such a poor family. Between his work, lessons and constant practice, studies took a back seat. After he dropped, he took a full time job, to better provide for his family and move up to a much more prestigious teacher.
This part is a little ~foggy~ still, but a few years after he left school, when he was around 18 or 19, his father caught him at the equivalent of "kissing the pool boy," and it was a short walk out the door from there. His mother wasn't nearly as furious, but she wasn't going to go against her husband, and they both agreed that it was best he didn't spend another night in their house, and not calling the police on him was the appropriate extent of parental mercy.
Fitz's sister, being the bro she was, had him live with her family in the year or two following this. She had two sons and a husband, who, though he was far better off than Fitz's father, still appreciated the extra money that would go to their family now, enough that he agreed to have Fitz stay. He was always pretty cold, though, because Fitz's sister never told him exactly what had had him kicked out of the family, and he always sort of assumed the very worst. A murder or something.
It was actually Fitz's teacher who was invited to Rapture initially, but after a short correspondence with the city about "Blah blah I'm too old to go, blah blah but my student would fit in great or you're always looking for more people," he secured a letter for Fitz instead. He headed down to Rapture at 20, and it was mostly smooth sailing after that.
Even though he was always the newest disciple, it was because he came to Rapture so late after it's founding. He pledged himself to learn under Cohen fairly immediately, and started working at the Fleet Hall not long after that.
Also he was pretty loose before he met Toasty. I don't think he's ever been with a woman, and he didn't really expect to ever have a long-term relationship, so he didn't see anything stopping him from living it up while he was still young enough to. v_v
And you all know the story from there. c:
Lol this was longer than I meant it to be.
I FEEL SO WEIRD WRITING BACKSTORIES FOR CANON CHARACTERS HNNNGH.