Sierra Sanchez is a girl Emily's age with short dark brown hair who lives in New Mexico with her uncle, where she helps run the Happy Trails Horse Ranch.
Her name was inspired by Queen Sierra from Princess Gwenevere and the Jewel Riders.
Sierra is a fan of Ravenswood, has been on the website, and was happy to meet the mages, Ozzie, and Dreamer. She also has a family heirloom jewel around her neck, an oval-shaped pendant of pure turqoise on a silver chain, which Emily suspects might be magical. During a horse ride, Emily tells her she thinks there is an animal nearby in need of help, but they don't find anything.
The next morning Sierra comes to the horse stalls to find Emily, Adriane, Kara, and a large number of, to Sierra's knowledge, horse colts and foals. The girls convince her to keep quiet, and Sierra is glad to give the horse colts and foals bushels of apples though she is confused as she swears she heard some of them speak.
Sierra's suspicions are confirmed when she goes to Emily and Adriane's cabin to fnd out the horse colts and foals are really baby unicorns. The girls again convince to keep quiet and tell her they need help getting them out of there without being seen. Sierra volunteers to lead the horse riders away tomorrow so the mages can sneak off to the Arrow Rocks.
When the time comes for the mages to leave, Sierra makes them promise to keep her updated on the events of Ravenswood and she promises to tell them if she finds anything magical about her jewel, though joking unless a herd of dragons show up, which the mages say is a possibility, giving Sierra something to think about.
Given the events of her debut, and her jewel, Sierra probably does have some magical powers. Her jewel is an oval-shaped pendant of pure turqoise on a silver chain, and proof she may have magic is the brief flash the jewel made that caught Emily's eye and Sierra was able to hear the thoughts of the freshman class of unicorn babies.
It is confirmed that Sierra does indeed have magical powers when she meets Tasha, the Goblin Sorceress, who tells Sierra her turquoise pendant has Healer Magic, with a little Warrior in it, her Element is Fire, and Sierra gains a Pegasus for a bonded. She first discovers her powers in Legend of LAOA.