It occurred to me that in the process of creating Harry's life, I created a whole load of OCs, including people that must have existed, but we know nothing about. So here's a guide to who's who in Harry's universe.
His father
A bit like an older version of Harry. He has pre-war ideas on how men should be and they should treat women. Which is understandable given that he was an adult by the time war broke out. He passed these ideas onto Harry. However, he does have rather Victorian ideas about family. His role in it is to provide, and as long as he's doing that he considers himself to be a good father.
His mother
Must have died at some point and, despite contradicting myself, I've gone with the idea that Harry was about 8, give or take a couple of years, at the time. She was in a coma for a while, so in some respects her death was something of a relief.
His stepmother
Goodness knows what she sees in Harry's father, but she's a fairly sensible woman. She's only ten years older than Harry and she becomes the person he goes to for advice. He talks to her about personal things more than anyone else. She was perceptive enough to work out what was going between Harry and his father and found ways to fix it without being heavy handed.
Will Sullivan
Is canon based on the Sarah Jane Smith audios. He's ten years younger than Harry and was about a year old when his mother married Harry's father. He idolised Harry growing up and followed in his footsteps in becoming a doctor and seeking adventure. He's known as Willie as a child, because Will isn't the sort of name you call a toddler.
Teddy Bland
Harry's best friend. They met at Dartmouth Naval College. They're both naval doctors, but have the same sort of attitudes and outlook on live. Although they don't see a lot of each other, due to not often being posted together, they remain friends.
Esther Bland
Teddy's sister. Harry met her when he was posted to Faslane Naval Base and she showed him around - she certainly knows the local area well because ten years later she's writing a book about it. Their friendship turns into something more once he accepts a transfer to UNIT. He goes as far as to think about marrying her, but just isn't ready. It turns out to be just as well because they drift apart after he joins UNIT (and leaves the planet, temporarily).
When they meet again, ten years later, she reveals that she's only really loved him, which touches Harry enough to consider marrying her. Sadly, though, she's quite bossy and overpowering and gets on his nerves. And by that point he's been in and out of love with Sarah and wants a different sort of girl from Esther.
Maria Carter
I named her after a character in The Bill (Marie Carver) and didn't consider until much later that there was a character called Maria in SJA. It was too late by then. Maria is a nurse that Harry met while it was in MI-5. They have a brief, but serious relationship, but she can't deal with him putting work before her. Their break-up is fairly acrimonious and she wants nothing to do with him ever again. Although Harry has regrets, he can't think of anything he'd have done differently.
Elaine Miller
He meets Elaine when he rescues her when he's working for UNIT again. She's a second school English teacher and for Harry, is the perfect mixture of bossy and submissive, despite being fifteen years younger than him. She was a lawyer earning a fair bit of money until accidentally she got pregnant at 29. When she changed her lifestyle to accommodate being a single mother she changed careers - and to one that doesn't leave her a lot of money.
Harry's determined to do everything right this time and they have a slow relationship. Right up until the time when he moves to Geneva and rather than putting off marriage, as he did with Esther, he proposes so that she can move out there with him.
Kelly Miller
Elaine's teenage daughter. Elaine kept her existence quiet for a long time, and Harry put off meeting her for a while. Eventually he met her when he rescued her from under an alien spaceship. Which helped their relationship rather a lot.
Kelly's a relatively normal teenager. She's never really worried about not having a father, but is quite happy to have Harry as a substitute. She likes drinking and partying at the weekend (even before she was 18) and she's quite independent. Unlike Elaine and Harry she's perfectly conversant with computers. She intends to spend her gap year travelling and then go to university to do French.