To me, this is something of an opinion question. I think that there are two characters who might fit the bill.
First, I think that you can say that Captain Clardy is the main secondary character. A lot of what happens to Jeff happens because of what Clardy does. Clardy's scam of selling the rifles has a huge impact on Jeff as well.
The other possibility is Lucy. Much of what Jeff does in the book (after he meets her) is dictated by the fact that he is in love with her. She continues to be important to the story after Clardy's part in the plot is done.