As Emma’s two romantic interests in the novel, Frank and Knightley become foils for each other, meaning their juxtaposition reveals truths about each character. Knightley is 37 years old, an esteemed gentleman, and someone Emma has known since she was born. Frank, on the other hand, is 23 years old, a playful flirt, and new to town.

While Frank’s jovial temperament at first makes Knightley appear dull or uptight in Emma’s eyes, after Frank’s secret engagement to Jane is revealed, Emma finds his previously fun and flirty behavior inappropriate and inconsiderate. This is the moment that it becomes clear (to both readers and Emma) that Knightley is mature and wise, while Frank is immature, re-establishing him as superior in Emma’s eyes.

Knightley’s reaction as he reads Frank’s letter to Emma explaining his behavior highlights the difference in maturity between them. Here he reflects on Frank’s decision to anonymously send Jane a pianoforte during their secret engagement (leading to much gossip and an overly full living room):

He […] stopt again to say, “the pianoforté! Ah! That was the act of a very, very young man, one too young to consider whether the inconvenience of it might not very much exceed the pleasure. A boyish scheme, indeed!”

In the end, that Emma elects to be with Knightley instead of Frank shows that she, too, has matured over the course of the novel, letting go of her misperceptions about Knightley and seeing Frank for the "boyish" man that he is.