Harry has Dumbledore. Luke has Obi-Wan (and Yoda), Frodo has Gandalf. Every good protagonist needs a mentor. It helps if that mentor has some pretty solid magical skills. She is a curandera (a type of medicine woman), but many suspect she is a bruja (a witch), and it's quite possible that Ultima's actually a bit of both.
Ultima introduces Antonio to the world of magic, but her most effective guidance is simply teaching him how to live his life and to become a man. She provides words of wisdom to Antonio throughout the novel. Here are just a few of the lessons she bestows upon him:
Ultima serves as Antonio's guide until her last breath. She uses her final words before she dies to bless Antonio. The last thing she utters to him is, "I shall be with you" (22.661-662).