- "I have yet to meet the man who could better me."
- —Agatha to Elizabeth Warleggan
Agatha Poldark was the great aunt of Francis Poldark, Verity Poldark and Ross Poldark. She had extensive knowledge of the Poldark family history and claimed to have witnessed six generations of her family, starting with her own grandparents.
Agatha was born in 1697. She was presumably a spinster, since she was known as "Miss Poldark", and probably served as housekeeper for some time until Verity came of age. She had a lack of contemporary family members as opposed to potential status as a widow.
Agatha frequently acted as though she was deaf, and while she may be hard of hearing, she certainly heard more than she let on. She was known for speaking her mind and guards many family secrets.
Agatha became very close to Elizabeth Warleggan, the wife of her late great nephew, Francis Poldark. She provided advice and company for her, particularly in times of trouble. However, she was unhappy about her second marriage to George Warleggan.
She accepted George's tendency for scheming and was unafraid of standing up to him. She supported her great-great nephew's abundance of rejecting George and became very close to him.
She adored Verity and was joyed when she would return to Trenwith. She knew the true parentage of Valentine Warleggan, something which she would hint at. She was treated poorly by George, who was unhappy by her presence and outspoken ways. She was a staunch protector of her family and was hurt when Ross would no longer come to see her at Trenwith, even saying he was no longer part of their family anymore.
However, she knew he and George were far from friends and was delighted when her great-great nephew informed her that Ross missed her, returning the same sentiment. She did not know that Ross had made a deal with George that he would stay out of his life provided that George treated her and Geoffrey Charles well.
She showed defiance against the idea that the Warleggan family name should replace the Poldark name at Trenwith. She hoped that her great-great nephew would return Trenwith to the way she thought it should be, even supporting him when he refused to change his name to Warleggan and stop seeing his extended family, telling him "quite right"!