Having struggled to make sense of Daisy's motivations and manners since they first met, Winterbourne finally accepts that she is unpredictable. The narrator uses a paradox to explain Winterbourne's view of her.
Winterbourne [...] very presently began to feel that she had no more surprises for him; the unexpected in her behavior was the only thing to expect.
Playing with the opposing words "unexpected" and "expect," the narrator uses this seemingly contradictory observation to encapsulate Daisy's character, as seen through the eyes of Winterbourne. On one level, the reader may wonder whether the unexpected becoming expected would make Daisy predictable after all. However, on another level, the paradox sheds light on Winterbourne's resignation to the idea that he will never perfectly understand Daisy's free-spirited, whimsical, and ambiguous nature.
The contradictory conclusion gives Winterbourne some peace of mind. In much of the narrative, he has forgone an active participation in his own life in favor of observing Daisy. He both commits himself to figuring her out and to figuring out why figuring her out is so important to him. He finally seems to get some respite from this undertaking when he realizes that, with regards to Daisy, the only thing he can know for certain is that she will always leave him feeling uncertain. Not only does he recognize that she will remain a conundrum to him, he also realizes that this is precisely why he is so fascinated by her.
Ironically, Daisy is more known to the reader than Winterbourne. If any of the main characters remain elusive throughout the narrative, it would be him. He may find her character inscrutable, but Daisy is a fuller character in the reader's eyes. Although he is arguably the protagonist, and although much of the narration occurs through his frame of mind, Winterbourne's character development seems to be paused over the course of the novella. His drawn-out attempt to understand Daisy prevents him from undergoing any significant character development.