Spirit Jr. attributes much of her physical appearance to her father, Spirit. She has a buckskin coat, black markings on her legs, a dark muzzle, a light brown mane and tail, and brown eyes.
Her defining mark is the blaze on her face, which she inherited from her mother, Rain.
Much like her father, Spirit has a headstrong and courageous nature. She loves to run with her herd and being out in the wilderness when not accompanying Lucky on one of her many adventures.
She hates being wrangled, and does not allow anyone to put a saddle or bridle on as she prefers to be ridden bareback. She does not appreciate being locked in a barn, as seen in one episode where she destroyed the barn that belonged to Pru's dad in an effort to get out.
Due to her experience with the wranglers, she also does not allow any men to touch her; an exception is Lucky's father following their treasure hunt adventure in "Lucky and the Mysterious Map".
Despite all this, Spirit is a very caring and loyal horse. Her intelligence and knowledge of the frontier proves very useful on her adventures, leading Lucky to trust her instinct over her own. Spirit cares greatly for Lucky to the point that only she is allowed to ride he, and she will come to her aid whenever she needs her help.
Spirit's care and concern for Lucky is demonstrated several times throughout the series, such as when she leads the group on a different path out of the canyon to escape the dynamite for the railroad, and when she begins lifting one of her front legs for Lucky to use as a way to get on her back when there are no fences or rocks to use.
- It is unknown where Spirit Jr. comes from or why and how she left her family.