PB is negotiable;
He is Diana Sharpe's best friend and Theodore Fairgood's (much) older brother;
He and Diana know each other inside out, and better than each realizes;
Anything else is up to you.
Kyle was the first born child to Norman and Alexis Fairgood, and as far as they're concerned, their golden child - even with Theodore's superior intellect, the fact Kyle's always chosen to follow in their footsteps and become an architect himself won great favor with the couple. Throughout his life he's come to agree that Theodore should have done the same, and due to that the brothers have always been at odds. The younger Fairgood's attitude certainly didn't help the lack of a strong brotherly bond.
When Kyle got to Tallygarunga, he soon made a friend who had been drafted into Bourke - Diana Sharpe. 'Friend' is actually an understatement - the two got along like two peas in a pod. It was this close connection that would make later years bearable for Diana. She was pulled from Tallygarunga by her parents at the end of their fifth year, forced to live in Mildura under a Muggle school system. The duo kept in touch nevertheless despite the blonde girl being forbidden to contact anyone from the 'magic world', and it was Kyle whom suggested her over time to manipulate her way into obtaining custody, wipe everyone's memory of her and her brother's existence, take him and run back. She did this two years later, just as Kyle himself graduated.
Two years after that (again), Diana finally graduated from Tallygarunga. Knowing her to be pretty much homeless at that point due to the stunt he convinced her to pull, he offered for her to stay in his family's home free of charge while she got through college and back on her feet. It was something she felt awful about, feeling like she was imposing, but ultimately took the offer due to having no alternative past giving up on her education. This choice worked out in Kyle's favor as well - a few months into things Diana introduced him to a new good friend she had made in college, whom then he proceeded to date. They have been together ever since.
Diana's been acting a bit weird ever since as well but keeping mostly to herself - quite literally in some respects, as she stopped dating altogether for example; however, now, five years later, she's moved out of the house and on her own, and a glaring ways away despite remaining in the same city. Apparition solves all issues, sure, but Kyle can't shake off the feeling she's trying to send a message and wanting to be left alone... But why would she want that?