Falling Off a Horse
The story opens with Penelope Featherington at age fifteen falling in love and knowing that Colin Bridgerton feels the same way. This thunderbolt of realization occurs as a result of an accident which starts with the bonnet she is wearing not being knotted tightly enough to stay atop her head and ends with Colin falling from a galloping horse. This series of events become the symbol of fate. Or, more precisely (and accurately) a belief in fate so wholehearted that it almost comes to pass simply by the force of that faith.
Penelope
Most of the entries in the Bridgerton series of romances have some sort of connection to fairy tales. In this particular case, the fairy tale is that of the ugly ducking who turns into a beautiful swan. Penelope—not exactly the most conventionally attractive woman in series—is the symbolic duckling in this interpretation.
Lady Whistledown
Without giving too much away, this discussion of the symbolic attachment of the novels to fairy tales must verge upon a spoiler. Suffice to say that Lady Whistledown is swanlike in symbolic importance.
Glue
Glue is used metaphorically several times through one particular chunk of the story. The glue in this case is symbolic of the Bridgerton family unity. They stick up for each other. They stick to each other. They are, symbolically—all of them—glue sticks that keep the family strong together.
Cressida
Cressida is beautiful. Cressida is popular. Cressida is graceful. She is, in a word, perfect. But perfection is boring and most people know that which is why most people who are perfect are also usually quite horrid. Cressida is the symbolic personification of this unsuitable paradox: tragically flawed perfection. Ultimately, Cressida is the swan who turns out to be an ugly duckling.