How would you describe Judith's character?

Let us start with some surface level characterization.  When Kit meets Judith for the first time, readers are definitely given the impression that young Judith is an exceptionally good-looking girl.  We are told in chapter 3 that Kit "gasped with pleasure" at the sight of her.  The word "perfect" is even used to describe her chin.  

Behind him a girl sprang up from the table and came forward. "This is your cousin Judith," her aunt said, and Kit gasped with pleasure. Judith's face fulfilled in every exquisite detail the picture she had treasured of her imagined aunt. The clear white skin, the blue eyes under a dark fringe of lashes, the black hair that curled against her shoulders, and the haughty lift of her perfect small chin—this girl could have been the toast of a regiment!

Judith is a beautiful lady, and she likes beautiful things.  She may be a Puritan, but that does not stop her from liking and desiring things of luxury.  

"What beautiful embroidery," she said admiringly.

The idea of having a big, new house to fill with fancy things is her dream. It is one of the reasons she has her sights set on William Ashby.  He can give her the dreamy life she is working so hard to secure, and we see her happiness at the end of the story. 

William's house on Broad Street was nearly finished. Piece by piece he was assembling the costly treasures for its furnishing-hue hand-turned bedsteads and chests and chairs from the skilled Wethersfield joiner, Peter Blinn, glossy pewter plates and a set of silver spoons from Boston, real china bowls of blue and white Delft from Holland. Judith knew where every piece would go in the new house, and how to care for each lovely thing to keep it shining. She and William spent their evenings in happy planning, and their contentment was good to see.

In many ways, Judith and Kit are very similar.  They are both quite headstrong and intelligent ladies. However, Judith has been raised in Puritan society.  She is much better at operating within society's limits as a result of her upbringing.  She is also a diligent and hardworking girl.  She might like and want things of luxury for herself, but she is much more practical and frugal than Kit.  Judith understands that in order to get something, she has to work for it.  She even has this attitude with men.  When the story begins, Judith is quite interested in William Ashby, but when he shows interest in Kit, she begins actively pursuing another man in order to secure a future.  She is not going to spend much time wallowing around feeling sorry for herself.  She is going to make something happen.