PB is negotiable;
He is Jamie Collins' and Alex Lin's estranged father, and Cole Lin's uncle. He left without warning/leaving a trace/ever coming back when the twins were 8, which doesn't make much sense, given he was living happily with his witch wife and children in Melbourne;
His wife never pursued him for divorce, the implication being she still loves him, and he didn't do it either so he might feel the same way;
Anything else is up to you.
A Melbourne native, Zachary was always jovial and trendy, even socially chameleon-like, and most saw him as the 'cool' sort - his wife certainly did, as she was smitten with him from the moment she saw him back when they were college students (in separate colleges). She quickly dropped everything to be with him, and it wasn't long after that the two brought Jamie and Alexa to the world. Each parent was closer to a specific child - Tabitha was closer to Jamie for he presented every quality in the book to become a wizard, and Zachary identified more with his daughter whom turned out to be a Squib. He put her in gymnastics and spent a lot of time with her in general, teaching her the basics of drawing at an early age and becoming the most important person in her life.
At a certain point in his life, when his little twins were 8, he was offered the opportunity for a big cartooning job a little ways away from their house in Melbourne; he took it, seeing it as a path for stable wealth and guaranteeing his children a future. What he didn't expect was for the work to turn out to be a convoluted scam when he got there, and it eventually crashed and burned a few months in, and along with it went all his investments (both monetary and in terms of time). Feeling like an idiot for falling for such an obvious scheme, he felt like he had no right or courage to face his wife after the whole thing. Sending her a note not to look for him, he went along with his life, starting from scratch and building himself back up from nothing.
Many years have passed. He's effectively still married, and has gotten back to his feet. Recently, he and a business partner decided to open a minor comic book publishing company in Melbourne. He hadn't realized he was back home until the day he happened to have to drive past what was his household once upon a time; and soon after, he found out his family didn't live there anymore. Perhaps it was about time he tried to redeem some of his past wrongs...