character changes

Hello, swimminchik, welcome to the forum.
I hope you read The Scarlet Letter; it certainly seems worth a good read.
To answer your questions, I would recommend visiting here, SparkNote's commentary on the novel.
Hester Prynne gets stronger mainly in her morals. Such as in the popular Friedrich Nietzsche quote "what does not kill me makes me stronger," she challenges the Puritan ethics of her time through her adultery, but, in the end, prevails in her situation, since Dimmesdale fathered her child.
Reverend Dimmesdale I do not think really gets weaker, but he ends up questioning his own beliefs through fathering Pearl, Hester's child. Through attempting to contain his "sinful nature," especially as a preacher, this practice wears down his morale, and even affects his physiological health. Eventually, however, his barrier between his secret and his surroundings breaks down, at his "weakness," so to speak.
Chillingworth, obviously, probably provokes the most evil in the novel. With one lie to conserve his identity, he commits another lie, then another lie, and so on, until he deceives everyone. With this, one could certainly say he "lives off his evilness" in a domino-pattern; each lie and element of deception only leads him further into another lie.