Master Spirits are a type of Spirit that cannot be equipped, and do not have classes and types like Fighter Spirits.
Master Spirits are Spirits that can only be obtained exclusively though World of Light, with the exception of Timmy & Tommy, which can also be unlocked with their Amiibo. Once unlocked, Master Spirits will run one of four types of facilities in World of Light. These facilities include Shop (which sell spirits, items, and Skill Spheres), Gym (which level up spirits), Explore (which bring back rewards), and Dojo (which teach styles to spirits). Master Spirits have dialogue boxes, though certain spirits (such as Kraid) communicate unintelligibly and therefore have provided translations. Master Spirits are considered Neutral types when used by puppet fighters in Spirit Battles. There are a total of 24 Master Spirits in the game. One can use the facilities from the menu without having to revisit them or entering World of Light after obtaining and entering them.
At the Gym, Spirits will level up as long as they remain assigned there. Arrows will indicate if Spirits will level up faster or slower.
When exploring, Spirits can be sent for treasure, with a chance of finding valuable items, SP, Gold, and the like after a given period of time elapses. The exact rewards vary depending on several factors, including the specific Explore facility used, the types, levels, and ranks of the spirits assigned there, and any styles those spirits might possess. Arrows will indicate if Spirits will level up faster or slower.
Dojos teach Spirits different Styles. A spirit with a Style active will alter its user's stats, boosting some stats at the expense of other stats. A Spirit can only know one Style at a time. Learning a new Style will cause the Spirit to forget any pre-existing Style. Previously forgotten Styles can be re-taught instantly. There are a total of 15 dojos.
Spirits can sometimes be bought from the Vault's Shop for Gold, but there are also five other shops that can be unlocked after defeating their corresponding Master Spirits in World of Light that use SP for payment. Each of these has a selection of spirits with a particular theme, and may also sell snacks, support items, and Skill Spheres. A few spirits can only be found in these shops.
A list of Spirits that can only be obtain from Shops can be found here.