The gentle Thomasin is the young innocent who through no lack of goodwill and right intentions on her part is treated roughly by circumstances or, in Hardy's view, by the sort of world she lives in. She is so normal and conventional in her views and her personality that it is easy to forget that she takes any part in the story. She wants to do right by everybody, is equitably and kindly treated by everyone but her husband, and by the end of the novel is conveniently disposed of for a happy future.
It is possible to suppose that Hardy added the sixth book only to make things come out right for her; at least, his readers seemed to have demanded it.
Her only fault, from her aunt's point of view, is that she persists in wanting to marry Wildeve even after she has not been well treated. But from the reader's point of view, this is a fault only in the sense that she is too generous in her attitude toward others, too willing to do the right thing as she understands it. So, in the novel, the innocent suffer too, though not irreparably.