I would say the costumes that least follow the period are those of the members of the Angel Club, which tend to be more modern, with occasional 30's flair. However, we don't see them too frequently, so it doesn't really spoil things overall.
|Sometimes, though, they nail it.|
|Double-breasted suits for all!|
|Please ignore creepy old man's groping hand.|
Plus, Ueno Rie and the Countess have some incredible wardrobes.
I am in love with Mok Dan's wardrobe. It's such a cute blend of tomboy and feminine, and perfectly incorporates the period feel, with suspenders, trenches, little neckties, and blazers.
Note—As this drama progressed, many of the costumes became more modernized, so I kinda stopped capturing them. They were still gorgeous, but Rie's costumes especially became less 30's fusion and more current with a vintage touch. (And sometimes not even that.)