Evil Is Sexy: Vidocq might be a villain, but he's easily the best-looking man in the game.
Genius Bonus: The game insists on hand-holding you through the dialogues as Sophie and Lautrec discuss each riddle and name more locations for you to visit to talk to people to help you figure stuff out. If you solve it yourself and come to the right location, in 95% of the cases the game will acknowledge it as being the solution, complete with a scene of Lautrec congratulating Sophie on figuring things out without his aid. The remaining 5% fall into the But Thou Must! category whenever plot exposition is tied into the location visits.
Hilarious in Hindsight Marie is a descendent of the French royal family. The 3DS game Rhythm Thief & the Emperor's Treasure, which came out about six months later and is also is set in Paris, has a character named Marie who is also a descendent of the French royal family. In addition to that she bears some resemblance to Sophie.
That One Boss: Sang Maudit. You have to fight him a total of two times, each with several phases. You don't get to heal or revive your spirits between the phases and can't replenish your supplies of healing items between the battles (you do, however, get to capture two more spirits in the interim, both of which he can one-hit-kill). Finishing the final form will probably burn not through all of your spirits, but most, if not all, of your gems as well.