Mystery is Maricela’s horse and Spirit’s love interest. Mystery is a very beautiful horse with golden color who became a decorated showhorse at youg age. At some point she ran away from her former owner Eliza Eaton when she tried to find Mystery a new owner.
After a bandit Samuel Hayes tried to steal her so he could sell her to make money but she managed to escape from him. She then met the PALs for the first time but ran away again. During their meeting, Spirit immediately formed an attraction to Mystery, which Mystery later returned when she got stuck in a thornbush were the PALS found her again and Spirit helped free her. Mystery ran away again, but secretly followed the PALs back to Miradero there she met Maricela for the first time and found a liking for her like Maricela did for Mystery, since they both shared a fondness for fancy things.
When Samuel Hayes showed up in town under the name Carlisle and convinced the PALs and Al Granger that he was Mystery's true owner. However, Mystery got scared upon seeing him and ran away, which made Lucky suspicious. When the PALS found her again outside Maricela's house, Maricela, with help of Turo, agreed to help keep Mystery hidden while the PALs searched for her true owner.
When Lucky found out the Eliza Eaton was Mystery's true owner she returned to Miradero to bring Mystery to Eliza. Maricela, having formed a a very strong bond with Mystery while hiding her, was disappointed to see her leave. This caused her to come after Mystery there Eliza Eaton at Diamond Gulch saw the strong bond and decided to let Mystery stay with Maricela.
Mystery later reappeard in Spirit of Christmas and had at this point became Spirit‘S Love interest, although the PALS remained oblivious. She even puts Spirit out of the way during a bison stampede, which shows how much she cares for him.
Mystery, according to Maricela, became sad when the PALs moved to Palomino Bluffs, mainly because Spirit had left, but the PALS and Maricela were once again oblivious to this. During the family weekend, Maricela brought Mystery to Palomino Bluffs for a reunion, where Mystery was happily reunited with Spirit and the other horses again. When Maricela realized that Mystery was in love with Spirit she decided to stay and become a student at Palomino Bluffs herself so the happy couple would not be torn apart.