Alexandra Keegan is born in January 1794 in Boston. She is Miles Keegan's oldest child, though he doesn't meet her until she's a few months old, because he's settled in Barbados and gotten married. Alex's mother, who she doesn't know much about except that she's American, named Mary, and was likely part Native American, was a "wild girl." In the 1790s, a lot could make you a "wild girl."
Miles, being a young, rich, single guy, had a regular mistress--Alex's mother--who he broke things off with before he moved to Barbados and married. Mary realizes she's pregnant, gives birth, then abandons the baby with Miles's best friend before disappearing herself. So when Miles comes to Boston to claim his daughter, he faces no opposition and has the baby christened, finally. Nick's been calling the kid "Alexandra" because his mother is named Alexandria, so Miles tacks on "Hannah" for his own mother and Alexandra Hannah Keegan is officially brought home to Barbados.
She turns out to be a loving older sister to Mady, but also a free spirit. Alex loves to ride her pony. She loves ships and sailing, like her father. She is willful, stubborn, and highly intelligent, but often restless. She's sort of the girl that the village gossips talk about, wondering who can ever get this girl to settle down and become a proper wife, although she's kind, sweet, hardworking, virtuous and intelligent. She's best friends with Mady and their stepsister, but also friends with many of the girls of the neighborhood. But being a tomboy, Alex is capable of running races, horseback riding, and swimming with the nearby sons of the local lord.
Later, after Miles dies, after spending a ton of her life in the English countryside, Alex is calmer, more ladylike, but still kind of a rebel for the time: she's actually good at math and while her stepmother is suffering from illness and unable to run the house and estate, Alex takes on the burden. She manages the finances of the house and estate, tells the housekeeper what needs to be done, and then even snoops around in her father's company's records. A woman of her time would have been expected to be able to run a household by Alex's age of 20 (through marriage), but it's comprehension of finances that makes her unladylike.
Alex becomes the maternal figure of her sisters. She meets a man at her guardian's mansion in 1814, an Irish-born army engineer who has brought his three sisters to their English relatives to be socialized and possibly find husbands.
While her sister Mady's main conflict is her mixed race background, for Alexandra, it's the frustrating position of intelligent women like herself at the time--that, and being illegitimate. Alex is in a comfortable position for a bastard daughter, she knows, but she always has that odd sense of having to prove herself.