servants are summoned by the holy grail into the human world in a physical form but they're also able to go into spirit form! they're varying levels of strong but all able to accomplish feats a human wouldn't be able to normally, but their existence is also tied to their masters/the grail and while a master can survive without a servant, a servant cant survive without a master (unless they take another option aka murdering civilians for it)

strong being, flickering eXISTENCE, THAT KINDA THING