Harriet is Tabby's mother and John's grandmother. She is sort of your stereotypical old-school, old-money WASP. She is extremely dignified and seems to live her life according to what she perceives to be the rules of society. She takes extreme pride in the fact that her family can trace its lineage back to The Mayflower, and she seems to look down her nose at people whose families don't seem to have the same rich heritage. Harriet initially scoffs at Owen and his family, especially because it's not as prestigious to come from a family who makes money by digging granite out of the ground.
After Tabby's death, though, Harriet seems to undergo a sort of thawing process. Even though she is still all about proper manners and good breeding, she stops assessing people by such rigid standards. Her relationship with Owen tips us off to this change. One of the funniest ways that their relationship shifts is through their mutual love of television and the flamboyant piano player Liberace. Harriet starts to respect Owen and even pays for the fancy clothes he needs to go to Gravesend Academy.