How do we begin discussing a hardened, old, kick-butt spinster like Marilla?
We're not going to—we're going to quote straight from a The Hairpin piece called "In Praise of L.M. Montgomery's Literary Crones,"
Marilla Cuthbert, at the beginning […] is all angles and sharp hairpins and even sharper words—a little advice; don't even think about touching her amethyst brooch—[…] From the outside Marilla might look starched and ironed and joyless, but underneath her petticoats she's a kindred spirit with a sense of humor and an enormous capacity for love. (Source)
In fact, aside from Anne herself, Marilla might be the woman who changes the most throughout the book. Here's how she's described in the beginning, before Anne enters her life:
She looked like a woman of narrow experience and rigid conscience, which she was; but there was a saving something about her mouth which, if it has been ever so slightly developed, might have been considered indicative of a sense of humor. (1.14)
With words like "narrow" and "rigid," she's the opposite of the imaginative orphan girl we meet in the next chapter. Much of the humor of the story comes from the back-and-forth between these two very opposite characters—Anne's long, flowery speeches peppered with Marilla's sarcastic replies.
But don't forget about that "saving something" of Marilla's mouth. The narrator is telling us that Marilla has potential to change…and to laugh.
And change and laugh she does.
As the woman who manages all the domestic tasks so her brother Matthew can work the farm, Marilla's immensely practical. She initially doesn't want to keep Anne because, she says, "A girl would be of no use to us." (3.31) When Matthew suggests that Anne could keep her company, she says,
"I'm not suffering for company." (67)
Marilla's doesn't see the point of filling an emotional need. She's all business, all the time.
But Marilla's also religious, and when she learns Anne's history, she sympathizes with the girl. She's not thinking of herself when she realizes Anne needs help. She decides to keep Anne because, she tells Matthew, "It seems a sort of duty." (3.32)
We can't blame Marilla too much for being curt and a little strict. Her job isn't easy. Suddenly, she's saddled with a flighty eleven-year-old who has no idea how to cook, sew, or pray—important skills in late nineteenth-century Avonlea. Matthew leaves the "bringing up" to Marilla, so she's stuck doing the hard stuff: punishing Anne and keeping her in line.
She starts out stern. Whenever she begins to feel tenderness toward Anne, she gets freaked out and falls back on what she knows—morals. She thinks morals are part of her duty:
Marilla was as fond of morals as the Duchess of Wonderland, and was firmly convinced that one should be tacked on to every remark made to a child who was being brought up. (8.42)
This comes from Marilla's religious background. A huge part of the responsibility she feels is to turn Anne into a God-fearing, humble adult with a Protestant work ethic.
What saves Marilla from sticking too hard to this goal is something mentioned in her initial description: her sense of humor. Sometimes Anne's so funny, Marilla can't help herself. When she catches Anne crying about the fear that Diana will marry and leave her someday,
Marilla turned quickly away to hide her twitching face, but it was no use; she collapsed on the nearest chair and burst into such a hearty and unusual peal of laughter that Matthew, crossing the yard outside, halted in amazement. When had he heard Marilla laugh like that before? (15.99)
Marilla's laughter's new, and it's changing her. Instead of trying to fix Anne, she starts enjoying her the way she is.
She can't say so, though. She won't even admit to herself that she cares about Anne until she sees Mr. Barry carrying her, hurt, from the Barry's yard and realizes that Anne "was dearer to her than anything on earth." (23.27)
Deep, deep down, Marilla's a total softie.
It might be easy to get annoyed with Marilla, if we only read her tart dialogue. But the narration gives us several peeks into her mind as well. We see how intensely she cares about Anne; we watch her give Anne a sensible goodbye and then weep bitterly into her pillow.
The other person Marilla cares about is Matthew, as shown by her unusual tears of grief when he dies. Because she's already vulnerable, that's when she finally tells Anne
"I love you as dear as my own flesh and blood and you've been my joy and comfort ever since you came to Green Gables." (37.21)
It's the first time she's verbalized her love. Marilla has changed from someone who thinks feelings are frivolous to a loving, emotionally open person.