We never see Susan Ann Sowerby directly, but she comes up in conversation every once in a while. She's the youngest Sowerby girl, at four years old. When Martha first meets Mary, she thinks that it's ridiculous that Mary has been so coddled that she doesn't even know how to dress herself. She scolds, "Our Susan Ann is twice as sharp as thee an' she's only four year' old" (6.14). This comparison is unflattering enough to get Mary to put on her own darn shoes.
Like Susan Ann, Elizabeth Ellen is another one of Martha's sisters who come up in conversation with Mary. When Mary thanks Martha stiffly for the jumprope that she brought her, Martha laughs a little at her formal good manners. She tells Mary that "If tha'd been our 'Lizabeth Ellen tha'd have given me a kiss" (8.36). As Mary begins to grow healthier and less weird, though, she gets more like Elizabeth Ellen (which is clearly a good thing). Mrs. Sowerby tells Mary, "Tha'rt grown near as hearty as our 'Lizabeth Ellen" (26.56). Yay.
Mrs. Medlock is Archibald's housekeeper. She brings Mary to Misselthwaite Manor for the first time, and she doesn't think much of her. In fact, Mrs. Medlock thinks Mary is as sour and unpleasant as they come. And since Mrs. Medlock is in on the secret of Colin Craven's existence, her standards for bratty kid behavior are probably pretty high.
Mrs. Medlock may not think much of either Mary or Colin, but she doesn't want anything bad to happen to either of them—she just doesn't want to manage their moods and deal with their weird tantrums. Once the kids start visibly improving in health for no (apparent) reason (she doesn't know about the Secret Garden or even about how much they're now eating), she becomes totally confused. On the other hand, we notice that she doesn't make much of an effort to figure out the mystery, since that would be too much trouble.
Mr. Pitcher is Archibald's personal servant. We don't know much about him except what Mrs. Medlock says to Mary: "Pitcher's an old fellow, but he took care of [Archibald] when he was a child and he knows his ways" (2.51). So there you have it.
Dr. Craven is Archibald's younger brother and Colin's primary doctor. Dr. Craven has often said that it would do Colin some good to get outside into the fresh air, but he always folds in the face of Colin's intense tantrums. None of Dr. Craven's medical recommendations—good or bad—seem to matter much in the face of Colin's bad behavior.
Since Colin doesn't trust Dr. Craven at all, Dr. Craven also doesn't have any idea why Colin is suddenly getting much better—he's certainly kept out of the Secret Garden. Like Mrs. Medlock, Dr. Craven doesn't fuss too much about investigating Colin's physical and emotional improvement. After all, his new health is keeping Colin out of Dr. Craven's hair.
There's one weird conflict of interest in Dr. Craven's treatment of Colin: If Colin dies, then Dr. Craven would be his brother's only surviving heir and the potential inheritor of Misselthwaite Manor. Still, the book acknowledges that Dr. Craven isn't a complete jerk—he recognizes that Colin's recovery would mean that he would "lose all chance of inheriting Misselthwaite," but even so "he did not intend to let [Colin] run into actual danger" (19.19). Dr. Craven may be a weak person but he isn't a villain.
Mary's parents both die in the first chapter of The Secret Garden. And it says something about her relationship with them that she never really thinks of her father (who was a British officer in colonial India) at all. As for her mother, the text tells us:
Mary had liked to look at her mother from a distance and she had thought her very pretty, but as she knew very little of her she could scarcely have been expected to love her or to miss her very much when she was gone. She did not mss her at all, in fact, and as she was a self-absorbed child she gave her entire thought to herself, as she had always done. (2.1)
In other words, we know that Mary's mother was beautiful, but we also know that she was so vain and selfish that she spent almost no time with Mary when she was alive.
How can Mary expect to care much about an almost-stranger, even if the two of them are close blood relatives? Mary's mother's death changes Mary's life only so far as she has to go to Misselthwaite Manor; Mary spends almost no time thinking about her as a person beyond the occasional reflection on her prettiness. That's probably the worst thing that you can say about someone: that her own daughter didn't really care that she died. Harsh.
Barney is the name of one of the two English officers who find Mary all alone and forgotten in her parents' house once they have both died of cholera. He seems more freaked out by Mary's lonely wait than she is; Mary sees him with a tear in his eye. Little does he know that Mary is about to head to a much better place than her parents' empty house (and we don't mean heaven; we mean Misselthwaite Manor).
Basil is the kid who comes up with the nickname of "Mistress Mary Quite Contrary" for our heroine. (See our "References" section for more on where this nickname comes from.) He's also the one who makes fun of Mary for not knowing that, when he says "home," he means "England" (2.10). But as we point out in our "Themes" section, it's totally weird to think of a country you have never seen as more home than the country you have been living in your whole life. Since Basil, like Mary, has grown up in India, we think this home business is a bit weird.
The Crawfords are an English family living in India who look after Mary until she can set sail for England and Misselthwaite Manor. They don't appear in the novel for long, but they do give us some insight into Mary's behavior. While Mrs. Crawford admits that she can see why her kids call her "Mistress Mary Quite Contrary," Mr. Crawford also notes:
Perhaps if her mother had carried her pretty face and her pretty manners oftener into the nursery Mary might have learned some pretty ways too. It is very sad, now the poor beautiful thing is gone, to remember that many people never even knew that she had a child at all. (2.16)
And of course, Mr. Crawford is quite right: A lot of Mary's bad manners come from her mom's total neglect.
The head gardener, whom Colin commands to keep out of the way while he is out in the garden with Mary and Dickon. Mr. Roach is a nice guy who laughs at Colin's high-handed behavior and obeys his orders. Mr. Roach's easygoing attitude toward Colin's bossiness helps the kids to keep their work in the Secret Garden a, well, secret.