Great-Aunt Sophy is the (somewhat tipsy) owner of the house where the borrowers take up residence. She employs Mrs. Driver, the cook, and Crampfurl, the gardener, and lets Mrs. May's brother stay with her to recover after getting rheumatic fever in India. Great-Aunt Sophy is bedridden from a hunting accident twenty years earlier (1.49), and her only comfort is drinking a decanter of Fine Old Pale Madeira every night.
But it's a good thing the lady gets a little tipsy every night, because she gets to talk with borrowers that she never would get to talk to otherwise. She looks forward to having some company in the evening, and begins a friendship with Pod:
"'She thinks my father comes out of the decanter,' said Arrietty, 'and one day when I'm older he's going to take me there and She'll think I've come up with the decanter too. […] Once he took my mother, and She perked up like anything and kept asking after her and why didn't she come anymore and saying they watered the Madeira because once, she says, she saw a little man and a little woman and now she only sees a little man…" (10.1).
Even though she thinks they are hallucinations, Great-Aunt Sophy begins a pattern of breaking down the prejudice in the novel between humans and borrowers. And we kinda dig her for it.
Eggletina is Arrietty's cousin, and Uncle Hendreary's daughter, who probably got eaten by a cat. She never appears in the novel, but we learn that she disappeared after going upstairs to explore.
Arrietty's parents use the story of Eggletina to deter Arrietty from going upstairs herself. Their plan backfires, however, because Arrietty just gets majorly jealous that her cousin had freedoms she lacks.
Arrietty projects her own feelings onto the character of her cousin, imagining that Eggletina simply ran away because she hated being denied her freedom:
"It's not that I'm afraid […]. I like cats. I bet the cat didn't eat Eggletina. I bet she just ran away because she hated being cooped up… day after day… week after week… year after year… Like I do!" (6.53).
Throughout the novel, the story of Eggletina is used either as a cautionary tale against exploring the human world, or as a way of finding freedom on your own terms.
Everybody's got at least one loopy aunt in the family, and Aunt Lupy is Arrietty's. She's also Arrietty's Uncle Hendreary's wife. We know she used to be a Rain-Pipe before she married a Harpsichord and became kind of stuck up. Homily is more than a little jealous of Lupy and her social status, and later starts saying "parquet" in the same snobbish way Lupy does.
When Arrietty sends a letter to her uncle and Aunt Lupy (who we never actually see in the novel), we discover that she has been missing for quite some time, but we never learn what happened to her. You'll just have to read the sequels to find out.
And now for our next trick, ladies and gentlemen, we will make Arrietty's uncle disappear!
Even though Uncle Hendreary is mentioned several times in the novel, he never actually shows his face. We know that he's Aunt Lupy's husband and Eggletina's father, and we know he was a good borrower until he was "seen" on April 23, 1892 by the maid (5.13). After Eggletina disappeared, he moved his family to the badger sets in the field.
Arrietty tries to get in contact with her uncle, and the boy returns with a poorly spelled letter, which says to tell Lupy to come home. The only problem is, none of the Borrowers know where Lupy is. We don't hear from Hendreary for the rest of this novel, but Mrs. May and Kate imagine that Homily, Arrietty, and Pod move in with the Hendrearys in the badger sets.
Ernie (no, not this Ernie) Runacre is the 21-year-old policeman who comes to investigate the "robberies" in Great-Aunt Sophy's house. Ernie doesn't have any actual dialogue in the novel, but we do know that in his childhood, Mrs. Driver chased him many times for "stealing russet apples from the tree by the gate" (19.25), and that she called him all sorts of nasty names. Mrs. May tells us that he doesn't believe Mrs. Driver's story of the little people who steal, which is not exactly surprising, given their shared past.
Rich William is the exterminator/rat-catcher that Mrs. Driver hires to drive out the borrowers. Mrs. May tells us that he is a dangerous man known as "the pig-killer" who had a "gun, a hatchet, a spade, a pickax, and a contraption with bellows for smoking things out" (19.41). This is not a dude you want to run into in a dark alley.
Mr. Frith is only talked about in the story, but we learn that he comes every once in a while from a place called Leighton Buzzard to wind the clock the borrower family lives in.
Rosa doesn't show up herself in the novel, but we know she is the maid who was dusting when she "saw" Great Uncle Hendreary on April 23, 1892 (5.13)—but who's counting?—and soon thereafter left Great-Aunt Sophy's employment.
Nellie Runacre doesn't show up herself in the story, but we know that she is Ernie the policeman's mother.