Tristran falls asleep under this tree, and wakes up to her talking to him. She gushes about the Lord of the Forest, Pan, who told her to look out for Tristran and give him a hand, er, branch. She also tells him that she used to be a "'wood-nymph. But I got pursued by a prince, not a nice prince, the other kind, and, well, you'd think a prince, even the wrong kind, would understand about boundaries, wouldn't you?'" (6.23). Apparently not so much, so to get out of the bad situation, she did some magic (or had magic done to her) and now she's a tree.
She's a chatty tree, and she's also pretty helpful. She tells Tristran that she will gladly help him because he showed the star kindness, and unchained her of his own free will. She guides him to Primus's carriage, where he hitches a ride and ends up in the same unlikely mountainous spot as the star. She also gives him a final piece of advice that helps him get out of trouble before the witch-queen can kill both him and the star. We like this helpful tree-person.