Casting
The next two important ones would be Geneva and William. At least chronologically.
Courage consists not in hazarding without fear; but being resolutely minded in a just cause.
I don't know as Geneva is all that important since she's going to be around so little. Maybe two eps tops.
William as a child - eh. William as an adult will be important.
I'd say some of the Ardsmuir men are more important since we'll be seeing them either from book 3 on or in later books. (ie Duncan Innes, Tom Christie, the Lindseys and couple of others I can't remember right off the top of my head).