Final answer:
Facial features, such as high cheekbones or a long neck, could suggest that Lucie is the Doctor's daughter if these traits are described as similar between them.
Step-by-step explanation:
The physical characteristic that tells us Lucie is indeed the Doctor's daughter can be inferred from descriptions hinting at shared or notable family traits.
While the excerpts provided do not directly mention Lucie or the Doctor, clues from descriptions of various characters might suggest that facial features, such as high cheekbones or a long neck, could serve to illustrate the familial resemblance. If Lucie is described with features similar to those, it could imply a genetic connection.