Demi-Servants (デミ・サーヴァント, Demi-Sāvanto?) are humans who have taken the Saint Graphs of Heroic Spirits into their bodies, functioning as Servants. While similar to Pseudo-Servants in the end result, the process of forming one is different.
As with other forms of Heroic Spirit Possession, each Demi-Servant is granted the abilities of the Heroic Spirit within them. One special characteristic is the Skill Possession Inheritance, granting them one Skill from the possessing Heroic Spirit. This skill is inherited by the Demi-Servant and sublimated in a self-taught manner.
Within the world of Fate/Grand Order, they are said to fundamentally be beings that should be unable to exist, similar to the concept of a vampire being unable to birth a mixed-blood dhampir. Mash is the only known one in existence, brought forth through various cruel experiments.
Within the world of Fate/Extra Last Encore, it is a process known to be able to be performed by taking the data of a Servant into the body of a Master. It is a process that can fundamentally only be performed between a Master and a Servant with which they summoned and formed a contract. It is said that they, no matter the sort of Master they are, will be unable to withstand the Saint Graph and will start to break down as time progresses. The only way to withstand the process would be if the individual was modified since birth.
|Mash Kyrielight||Shielder||Galahad||Grand Order||Fate/Grand Order|
|Rin Tohsaka||Lancer||Cú Chulainn||Moon Holy Grail War||Fate/Extra Last Encore|
|Rani VIII||Berserker||Lü Bu||Moon Holy Grail War||Fate/Extra Last Encore|
Characters that have similar characteristics to Demi-Servants but have not been officially classified as such.
|Shirou Emiya||Archer||EMIYA||Fifth Holy Grail War||Fate/stay night: Heaven's Feel|
|Sieg||Saber/Berserker||Siegfried/Frankenstein||Great Holy Grail War||Fate/Apocrypha|
 Fate/Extra Last Encore - WORLD/WORD: Demi-Servant
One who has taken a Servant's data within his body, inheriting his abilities.
Fundamentally, this can only be established with a Servant one has summoned and formed a contract with.
However, one cannot withstand the Saint Graph (spiritual foundation) of a Heroic Spirit no matter what sort of Master he may be, and supposedly will break down along the way.
It may be possible to withstand this if the individual has been modified for this from birth, but....