This character is morally strong enough not to be persuaded to do anything wrong: his way of thinking will not deteriorate and will not be destroyed by anyone, no matter how difficult his experiences.

This trait can be used (for example) for a character who can not be corrupted by money or for a character who is violated (by proxy or otherwise) by others but who would remain "loyal" to his lover and do not succumb to pleasures given of his rapist.