This is an interesting book from Heyer. On the face of it, Jack Westruther is much more her typical hero type - handsome, witty, quick on the uptake - but while not unsympathetic to him, in Cotillion she explores the character traits that make this type perhaps not such a suitable long-term match.
As a contrast, Freddy Standen is about as far removed from Heyer's dangerous archetype as it is possible to get. Not, on first acquaintance, particularly quick on the uptake in the conventional sense, the reader, along with Kitty, soon realises that he is very far from stupid. Importantly, he is a genuinely good person, motivated by the desire to do the best for Kitty and to help her out of the scrapes in which her involvement in the affairs of her cousins and her friends rapidly lands her, and his normally restrained demeanour does not prevent his being roused to passionate emotion.
Kitty Charing is, in her turn, is a delightful heroine - only nineteen (and believably so), raised in rural seclusion with an overdependence on sensational works of literature, she is determined to achieve some measure of independence and gradually learns that what she needs from a man is very different from the romantic heroes of poetry and legend.
In addition, the sub-plots - Freddy's extended family and Kitty's connections, Kitty's slightly dodgy French cousin Camille, her beautiful but impoverished friend Olivia, the feeble-minded but good-natured Lord Dolphinton and his distinctly "trade" inamorata - add sparkle and charm to the narrative and evoke genuine interest in the interwoven plot strands.
Overall, this is one of my favourite Georgette Heyers - and I speak as one who is normally a sucker for her more witty, quick-tempered, and worldly heroes (as in Devil's Cub and so forth). This book may have more of a nice cocoa-ey feel to it, but it's none the less charming and engaging for all that, and I defy anyone not to buy into the general warm fuzziness at the end as Jack gets his comeuppance. Also some genuinely laugh-out-loud lines. Definitely recommended.