I want to say it's shown up elsewhere post-GO too but it definitely started in GO.
I used to think it was just a name for the Servant vessel things, like "oh this temporary body created by the Grail to shove a piece of a Heroic Spirit into is called a Saint Graph", but then they started using it for stuff besides Servants.
I'm not looking at events but in the main story the earliest I can find to using it for non-Servants is the Lion King in Camelot.
Also in the climax Romani says Bedivere does not have one so normal living humans don't.Dr. Roman: It was King Arthur's knights, the Knights of the Round Table, who annihilated the Crusaders and built a Holy City upon the Holy Land!
Fujimaru 1: The Arthur we met in Singularity F!?
Fujimaru 2: The Arthur we met in London!?
Dr. Roman: No, this one's different.
Dr. Roman: The Spirit Origin output observed from that Servant just now was far beyond that of King Arthur in our data!
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Wait actually I didn't look at the context of my ctrl+F-ing, I saw "Spirit Origin" and "beyond King Arthur" and jumped the gun. Roman is talking about Gawain there. Disregard me.