Helen is of the moralistic novel variety, and while it is more character driven than typical of the style IMO, the characters do serve (and suffer a little from) the overarching point of the story.
The length got a little tedious, I must admit, but it did serve to illustrate the value of truth and honesty, even when falsehood seems necessary for the happiness of the deceived party, in reality it only causes more pain and distrust.
Helen is one of those angelically virtuous heroines that are a little hard to believe. She is entirely lovable and so unselfish she continually allows her friend Cecilia to take advantage of her, eventually to the agony of both. Cecilia is one of those good people without strong moral character who take the easiest route through life, lying whenever the truth seems like it would cause pain, much to her gradual sorrow.
The heroes of the story, Granville Beauclerc and General Clarendon are rather severely underdeveloped. Both are too good to be true (they do have their little faults, though they seem tacked on for effect), and spend the entire story urging Helen and Cecilia to be frank and truthful. Honestly, while I like him, I never saw much in Beauclerc. He's headstrong and too fanciful for my taste, and Helen is a little flat. Other than her goodness, about all we know of her is that she plays the harp, has handwriting almost identical to Cecilia, and is a pushover.
Lady Davenant is the most interesting character of the book in my opinion. She is certainly the voice of reason throughout.
As I said, the characters serve the theme, and their personal quality is low because of it. The contrasts between characters, and the teaching moments created by them are their great purpose. The good are very good, and the bad are very bad.
Helen is worth reading as a classic, and also for it's thought provoking content, but don't go in expecting plot, or Austen quality wit and commentary.