The novel's main character is Henry Smith.  He is part of a wealthy Massachusetts family.  Sounds good so far, except that Henry's father has tried to keep his family as far away from trouble as possible.  One way he does that is by giving his kids every material possession they might need or want.  Unfortunately, dad gives his kids stuff, but not much emotional love. That is one reason why Henry struggles so much with the death of his brother Franklin.  Franklin and Henry had planned on climbing Katahdin together, and Henry vows to climb it alone in honor of is brother.  It's his coping mechanism, since mom and dad aren't much help.  

Henry plans on climbing alone, but is unexpectedly joined by 3 others.  Sanborn Brigham (his best friend), Chay Chouan (an immigrant), and Louisa (Henry's sister). That's who Louisa is -- she and Henry are brother and sister.