One event on Pride and Prejudice leads us to see the principled side of Darcy: He paid for Lydia's wedding to Wickham.
Lydia- who was quite foolish- eloped with the deceitfull Wickham without getting married first. This faux pas would have cost the entire family its reputation. Yet, as much as Wickham and he detested each other, Darcy came forward. Indeed, the whole thing with Lydia was shameful to the Benets. Darcy put this whole thing down on writing to Elizabeth in a letter aiming to appease her from her ill feelings for her.
Hence, Darcy does have a principle that dashes away from his superficial "rules" of "not dancing", or "not liking plain women"-like Elizabeth, and yet- they ended up together.