A character essay is intended to get inside a character's head, and show subtleties or hidden nuances that might not be immediately obvious. There is a lot of information about the correct writing of essays online: here is a slideshow about character essays, and here is the Purdue Online Writing Lab, which is a good source for formatting and citation information.
For The Witch of Blackbird Pond, your first step should be to read the book all the way through; this should be obvious. Secondly, think about which character stayed with you and made the biggest impression; you do not need to focus on the protagonist, especially if another character seems more interesting. For example, Goodwife Cruff, as the antagonist, would make an interesting analysis, as would Thomas Tupper, who is dead but is referred to in the text. You can also judge if any of the characters have a connection with the author, Elizabeth George Speare; if so, you can write a powerful essay on the author as avatar in text.
Remember that you do not necessarily need a thesis statement for this sort of essay; check with your teacher. If you do, a simple one will suffice: "Captain Eaton is a firm leader but has a kind heart." The most important thing is to support any statements you make with quotes from the text; see below for the Google Book preview, where you can find a limited amount of text online.
Remember that the most important factor in a character essay is that the reader connects with the character through your description. A good essay will allow a person who has never read the book to understand the character as well as one who has.
I think I can help answer your question. Deciding which character and trait would be easiest to write about really depends on how you responded to this work as a reader.
Start by thinking about which character you connected with on a personal level. As the protagonist, Kit is a natural choice because you know the most about her. However, take some time to consider other characters you found intriguing.
Once you identify the character you want to write about, think about the most memorable scene in which this character was involved. Which traits are most dominant in this scene? How do the actions of this character influence what takes place in this scene?
I hope that this helps.