She lives in England with her parents and brothers. The life she leads is largely unknown, but implied to be average.
Along with the rest of her family, she visited Danville to see their favorite association football team, the Snifferton Nostrils in an exhibition match and presumably stayed in the Flynn-Fletcher household with their cousin, Ferb during that time. At the request of her step-cousin, Candace, she taught the American how to be a properly civilized lady so she could impress Jeremy Johnson after having found out of him being an Anglophile.
Candace learned the ropes quickly and shortly afterwards took on the personality and behavior of a proper Englishwoman when she later went out with Jeremy. However, she ditched her training when Jeremy made known that he liked her for her usual attitude. ("My Fair Goalie").
Having been trained into the person she is from an early age, Eliza has a reserved personality and is very elegant. She displays a patience that is rivaled by few, as seen in her attitude towards Candace when the latter panicked about Jeremy complimenting Eliza's accent and not hers. She showed absolutely no sign of surprise upon witnessing what Candace's brothers are capable of. In Candace's view, Eliza is very "lady-like" and has very good manners.
During Candace's lessons, Eliza demonstrates an aggressive side of herself, her classical musical number shifting bluntly into a punk rock song. Since this happens particularly when she discusses what ladies shouldn't do, it is somewhat reflective of her namesake, Eliza Doolittle, whose true personality would show itself under her appearance of refinement.
Her appearance contrasts from the other members of the Fletcher family, although she shares Candace's head and neck shape despite the two not being blood related. Eliza has brown hair, which she wears in a short ponytail. She is slightly shorter and thinner than Candace, but they appear to be around the same age. As for apparel, she wears a pair of black shoes along with a pair of blue jeans which are rolled up. Both are topped by a white buttoned-up collar shirt with a brown belt and a black jacket. When she trains Candace to behave like a proper lady, she sports a blue Sunday bonnet hat, a long blue dress with light and dark striped blue bows, white gloves, and her striped sash.
Candace is Eliza's step-cousin. Eliza is patient with Candace's obsessive behavior and willingly teaches her the basics of British etiquette. She seems to like Candace in her typical disposition and isn't surprised when Jeremy reveals to both he does as well.
Ferb is Eliza's cousin and Phineas is her step-cousin. Since no direct exchanges are seen between her and the boys what she thinks of them is largely unknown. However she doesn't seem to have any disagreement against them. When Candace sets out to bust her brothers, Eliza stops her and tells her that self-restraint against acting in retaliation of the shenanigans of younger siblings is important in showing discipline as a lady. She doesn't appear to be surprised with the size of their feats.
Being her father's brother, Lawrence is her uncle. She appears to like him as a typical niece would. However, they are never seen interacting directly with each other, besides that she upheld a cheerful disposition as he introduced her to the Flynn family.