Although Franz d'Epinay is the only character in the book who gets to go on a really intense hashish-induced trip, he's arguably the most reserved one as well. In Rome, he's the reasonable counterpart to the careless, carefree Albert de Morcerf. His aloof air definitely has something to do with his aristocratic upbringing – like his friend Château-Renaud, he's part of the old "aristocracy of the sword."
His noble bearing makes him a bit boring, though, and not surprising that Valentine Morrel prefers her globetrotting army officer to the considerably wealthier d'Epinay. Heck, even after learning that Noirtier killed his father, all he can do is issue a strongly-worded letter of refusal.