A designation given to an individual, usually a Magus, who has formed a contract with a Ghost Liner called a Servant as their Familiar. The Master provides the Servant with Prana and a sort of “passport” into the modern era in return for the services of said Servant.
In order become a Master, a person must successfully summon a Servant through use of a specific Formal Craft ritual. Upon completion, the Master must formalize a contract with the Summoned Servant by acknowledging their respective roles. The process creates a link between the two, allowing the Master to support the Servant with their Prana or an external source.
Masters only occur through the power of the Great Grail of Fuyuki City. The Grail both selects prospective Masters according to their need and provides the majority of the energy needed to manifest and maintain the Servant. It also provides the Command Spells which grant limited control of the Servant to the Master.
Under ordinary circumstances, a Master is always a spellcaster, usually a Magus with a Magic Circuit, who is selected by the Grail. It is possible for an ordinary person to become a Master, as well as untrained individuals, such as Ryuunosuke Uryuu, though this is rare.
A Master that is providing Prana to a Servant will “leak” magical energy in a volume that is easily detectable to most spellcasters. The leakage is distinctive, allowing a Magus to determine that the Master has a contract with a Servant.
An active Master will also possess a Command Spell as visible proof of their contract.
A Master is always a Master, whether he currently has a Servant or not, as long as he still has the Command Spell. A Master without a Servant can make a pact with an unclaimed Servant at will, though having too many Servants is hazardous to the health of a Master due to Prana consumption. This also means that a Master can claim a new Servant regardless of the length of time since his last Servant.
When a Servant is summoned, it normally matches the personality of the summoner. This is a form of catalyst, in which the Master and Servant share similarities and naturally gravitate to one another. However, it is possible to summon a specific individual by using an object linked with that individual in the summoning. The more closely the item and person were associated, the higher the probability of obtaining that Servant. This allows targeted summoning of especially strong heroes, but with the risk that the summoned Servant will be ideologically/morally incompatible with its Master, thus lowering their efficiency in the Grail War. It is also possible, in rare cases, for the Servant to possess a catalyst associated with the summoning Master, allowing the process to work in reverse.
A Servant without a Master can form a contract with a normal person without a Magic Circuit through the sharing of Vitality in the same manner as certain types of other Familiars. The Master must still find a way to provide the Servant with Prana, however.