The fact that the question contains the words "thus far" I will assume that you have not read the entire book yet.  This means you only have to look as far as you have read to find examples.  In order to complete this assignment I encourage you to follow these simple steps in order:

1. Choose a character. (The easiest, in my opinion, are either Reuben or Swede, though Jeremiah would also be fun.)
2. Make a list of examples for your character following the characterization guidelines above (speech, action, reaction, etc.)  If you found just one example (a quote or paraphrased idea) for each, you would have a great start.
3. Examine your list.  From it, try to draw a one-sentence conclusion about your character.  This is where you will include a few pointed adjectives to describe who he or she really is.
4. Your one-sentence character "summary" will become your thesis statement.  Then, you can prove the thesis using the examples you brainstormed in step #2.

A couple of things to consider:

Reuben is the narrator.  Most of the insight you will glean from him comes from his easy way of speaking and story-telling, the personal connection he has to each member of his family, and his imagination.  Consider that he is young when he experiences the story, but tells much of it from a grown up perspective.  You might also want to include the fact that he should have died as a baby, but didn't, and thus believes his father works miracles, and his life has a purpose.

If you choose Swede, her writing and poetry throughout the story should not be ignored.  Much like her brother Reuben, she is imaginative, intelligent, and acts much older than her age.  Also consider that she is the only female, being raised by a father and two older brothers.