Frederick’s dimple is a symbol of Cassie’s romantic feelings for Frederick. When the dimple first appears, it’s when Frederick smiles at Cassie during their first meeting: “When he gave me a small, expectant smile, a dimple popped in his right cheek” (22). Cassie notices the dimple immediately, in the same moment when she notices how handsome Frederick is and becomes attracted to him. Later, when discussing the truth of his past, Frederick smiles at Cassie again, causing her to think, “I refused to be distracted by how handsome he looked when he did those half smiles. Especially when they made his dimple pop” (127). 

Cassie tries to focus on what Frederick is saying and on staying neutral in their conversation, but the physical presence of his dimple reminds her of her romantic feelings for him, even after he broke her trust by keeping his vampirism a secret. Her feelings for him only intensify as they grow closer. When Cassie takes him shopping for modern clothes, she finds him attractive in the clothes he tries on and tells him so. This makes Frederick bashful, which only makes Cassie find him more attractive, as she notices, “​​He gave me a bashful, lopsided smile that activated his killer dimple, then looked down at his arms” (175).