In the novel The Mystery of the Blue Train, The Hon. Lenox Tamplin is Lady Tamplin's daughter. Her father was the late Viscount Tamplin. She lives at the Villa Marguerite.
Lenox is described as being a "sad thorn in Lady Tamplin's side". This is because she has "no kind of tact", looks older than she actually is, and has a "peculiar sardonic form of humour" which Lady Tamplin finds uncomfortable. Lenox has a particular sideways smile, which Lady Tamplin finds difficult to understand. She is also outspoken, and has what Lady Tamplin calls an "uncomfortable way of putting things".
When Katherine Grey first arrives at the Villa Marguerite, Lenox observes that Katherine is becoming restive due to Lady Tamplin's constant questioning about the murder on the Blue Train. Lenox takes Katherine upstairs to see her room, and tells her not to mind Lady Tamplin.
Lenox loves clothes, but is of the opinion that clothes are no good to her, because she was "born awkward". She admires Katherine's clothes, and the two of them are able to discuss clothes with "artistic fervour".
Lenox tells Katherine that everyone "goes off their head about" Derek Kettering, and that she herself sometimes does as well. However, at other times, she thinks that she would like to marry a curate, live in the country, and grow things in frames. She is of the opinion that an Irish curate would be best, so that she could also hunt.
Lenox and Katherine become friends while Katherine is staying at the Villa Marguerite. While Lady Tamplin tries to exploit Katherine's connection with the Kettering case, Lenox adopts a detached attitude, seeming to be amused by her mother's actions, but also understanding of Katherine's feelings.
After Derek Kettering is arrested, Poirot goes to the Villa Marguerite to tell Katherine about it. Lenox receives him, and he tells her the news, to pass on to Katherine. Lenox expresses the opinion that Ruth Kettering need not have been killed by someone on the train. She says that someone might have swung onto the train when it was at a station, killed Ruth and stolen the rubies, and then dropped off the train again.
At the end of the novel, Poirot tells Lenox that her comment about the murderer not needing to be someone on the train helped him to see the truth.
Lenox had been in love with Derek Kettering, but eventually concludes that she is too young for him, and it would not have done. She expresses the opinion that she will never find love, but Poirot tells her that she is still young.