Servants.

What mages(we) call Ghost Liners.

We have various ways to use them. For instance, we can summon them from their “throne” for a short period of time using a ritual to use certain parts of their power or certain fragments of their Noble Phantasm.

However, only the Grail in Fuyuki can summon a servant along with their personalities. Or at least, as far as I know.

Lord El-Melloi II's Case Files: The Atlas Contract; Chapter 1, Part 3