Lavinia Lyte, usually referred to as Miss Lavinia or Miss Lyte, is the most beautiful woman in Boston—and she knows it—and Johnny's mother's first cousin.
Lavinia Lyte is a spoiled, jaded rich girl, and as her father's only surviving child, she pretty much gets what she wants whenever she wants it. She also likes to make an entrance, which we see both when she disembarks at the wharf and when she arrives at Johnny's trial. She makes another dramatic entrance much later when she arrives to tell Cilla that Mrs. Tweedie (formerly Mrs. Lapham) is allowing Isannah to go to London with the Lytes. In short, Lavinia Lyte may not have a professional acting career in her future, but she loves attention.
She is a foil for Cilla in that very little about her is really genuine. Her blazing beauty is all a front for something that is really nasty: "In his heart he knew there was something unlikeable about Lavinia Lyte. Mrs. Bessie knew what it was. And Cilla now knew. But she wouldn't tell him. Now, when he wanted to talk about Miss Lyte, she would veil her eyes and look at him from out of the corners, and say nothing. But he knew Cilla knew" (7.5.3). Despite this awareness, Johnny continues to be dazzled by Lavinia Lyte's beauty.
Lavinia's not all bad, though. She genuinely loves her father—their love for each other may be the one redeeming part of both their characters—which we see when she throws herself into his arms upon her return from London, and also in her concern for him after the attack at the Milton house. Even at the end, she tries to protect her father's reputation by telling Johnny that Merchant Lyte didn't mean to cheat him. She's also curious and fair with Johnny—once she finds out the truth of their relationship—and it's only through her investigations and her willingness to share those results that Johnny finds out about his parents at all.
But in the end, she values money and comfort and getting what she wants over all else, which is partly why she returns to London to marry a lord. Still, the other reason she leaves is to take care of her ailing father, so…again, she's not all bad. She's a tricky one, that Lavinia Lyte.