The Drama Queen
Isannah is Mrs. Lapham's youngest daughter and Cilla's little sister.
We'll say this for little Isannah Lapham: she makes a big splash. Whether she's saying the catechism to clergymen in exchange for candy or throwing herself on the judge at Johnny's trial, she's hard to miss. We see her first as a relatively innocent, if vain, eight-year-old, but she soon transforms into the most sexualized character in the novel. Kind of weird, when we consider that she's only ten by the end.
We see Isannah first as the object of Cilla's affection. Whereas everyone else in the house thinks she's more trouble than she's worth, Cilla humors her through her real or imagined illnesses, buys her hair ribbons, and generally thinks she can do no wrong. It's a testament to Cilla's steadfast care for others that she puts up with Isannah—and adores her—no matter what. Isannah is a foil for Cilla: Cilla cares for others, while Isannah only cares about herself.
It's after she falls under the corrupting influence of Lavinia Lyte that Isannah's real transformation begins, and it's not long before she's rolling on the drawing room floor in her underwear in front of half His Majesty's Finest. When she bats her eyelashes at Johnny and tells him, "I'm too young to be lascivious" (7.5.14), he slaps her because oh-no-she's-not.
It's significant that Isannah is trained to play Nell Gwyn, the famous mistress of Charles II, in an entertainment for Miss Lavinia and her suitors, and that Miss Lavinia plans to put her on the London stage. In the eighteenth century, there wasn't much difference between an actress and a prostitute as far as Polite Society was concerned. And remember Isannah trading catechisms for candy? Take that down the slippery slope.
It's worth pointing out that Miss Lavinia plans to turn Isannah into what she herself would have been if she had not been born wealthy; because Isannah is poor, she is vulnerable to Miss Lavinia's machinations. Miss Lavinia is every bit as flirty and attention-seeking as Isannah, but she can afford to be—her position means that she'll never need to work as an actress (or a prostitute), and her reputation is far more protected than Isannah's is.
At the end of the novel, Isannah chooses to go to London with Miss Lavinia rather than remaining in Boston with Cilla. "Izzy is no good, thought Johnny. She'll go" (11.4.21). Isannah has already gone so far down the road of corruption that she might as well take it all the way back to London rather than staying with people who are fighting to form a new type of society.