Lettice is described as a "pretty girl, very tall and fair and completely vague". She appears to be dreamy, and seems to like people to think she is unintelligent. However, she is actually more intelligent than she lets on.
Lettice does not get on with her father or her stepmother, Anne Protheroe. She was upset with her father for forbidding Lawrence Redding to come to the house, because he was painting Lettice in her bathing dress. She also felt that her stepmother hated her.
On the day of the murder, Lettice attends a tennis party in Much Benham, together with Dennis Clement. She leaves the party early, which causes Susan Hartley Napier to say that she has rotten manners.
Lettice later searches the study at the vicarage, hoping to find something that would incriminate her stepmother. When she finds nothing, she deliberately drops an earring belonging to her stepmother. She says that she did this because she knew that her stepmother had killed her father.
At the end of the novel, Lettice reveals that Mrs Lestrange is her mother, and that she had left the tennis party early to meet her. She also says that she destroyed a portrait of her mother in the attic at Old Hall. She did this because she was afraid that her mother would be suspected of killing her father.
Lettice tells Reverend Clement that she is going abroad with her mother, who is dying, and that she will be with her mother until the end.
In the ITV 2004 adaptation of the novel (Episode 2, Season 1 of Agatha Christie's Marple), Lettice is played by Christina Cole.