Solo Servant

As part of SE.RA.PH’s reconfiguration, it gained an additional function: the ability to randomly summon Servants as part of its plan to develop the world.
It generally prioritizes Servants who had performed an important role in the Holy Grail Wars, but there are some Solo Servants who are not in the records of these Wars.
They can act independently and are powerful even without a Master, but ultimately not having a Master is hard to make up for. An NPC or Al can become a Master so long as they have an independent thought routine, a properly designed soul, and enough magical ability, but a Servant performs best when paired with a human who has a body made of pseudo-spiritrons.
At the start of EXTELLA, the only Servants who are not Solo Servants are Nero and Tamamo no Mae.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA