Upright the Knight of Cups is read as romance, charm and the pursuit of an ideal, imagination in motion; reversed, as moodiness, unreliability, or charm that has curdled into manipulation.
As a person card, he is conventionally read as an idealistic or artistic figure, someone who leads with feeling and imagination rather than with plan or force, and readers often distinguish him from the King of Cups on exactly this point: the King has mastered his emotional register into something steady and self-controlled, where the Knight is still actively pursuing, still riding toward something rather than having already arrived. The distinction matters practically, since a querent asking about a specific person benefits from knowing whether the reading points to pursuit or to arrival.
Waite's own text describes the upright card as arrival, invitation and proposal, a reading with a more concrete, event-based flavor than the purely dispositional romance and imagination most contemporary readers now emphasize; the shift toward reading the court cards as personality types rather than as events or messages is general across the deck's twentieth-century reinterpretation and is not unique to this card.
Reversed, the same idealism that makes the knight appealing becomes its own liability, an ideal pursued so single-mindedly that it stops noticing whether it is wanted, or a charm deployed less as connection than as a means of getting what is wanted from someone else.