Because Angel's initial rejection of Tess after her confession leaves her in anguish, we could almost list Angel under the category of "antagonist," but that would be over-simplifying things. It would also be a misreading of Tess's relationship to him. Even after he's gone, she looks to him as a guide and mentor. She has internalized everything he's ever said to her, so that she's even begun to speak as he spoke. For example, she's able to repeat, word-for-word, everything that Angel said to her on the subject of religion when Alec asks her why she doesn't believe in sudden conversions. Even though Angel is clearly at fault when he leaves her, he still acts as a kind of "guardian angel" to her in his absence.
The difference between social laws, and natural law, is one that is continually brought up in this novel. You should go to the "Contrasting Regions: The country and the city" theme to read more about this. "Nature" acts as a guide to Tess when no one else is around to help her. She crawls into a hedge to sleep one night after being harassed by a man on the road, because nature is a safe haven, and "outside humanity, she had […] no fear" (41.27).