Why does Huck tell Jim he won't turn him in, when he is so frankly opposed to abolition? What does this reveal about Huck's character?

Huck promised Jim he wouldn't tell, and Huck does not break his promises.  In addition, Huck has been told to oppose abolition when that might not be his actual opinion.  Huck is also very lonely when he is by himself.  Jim is Huck's company and keeps him sane.  This tells the reader that Huck can not be alone, and he is a trustworthy person.  Also, it shows that Huck has been told what to think about slavery.