EMIYAman
I BONE OF THE SWORD
"Aren't Heroic Spirits, and most likely aren't from the Throne of Heroes." I got chills thats a plot point if I've ever seen one, you can use that.The only plausible way I could see fictional characters in a Holy Grail War is if Zelretch became an internet troll, acquainted with web culture, then got really, really drunk and decided to raid Fuyuki to claim the Grail. Cue several modifications to the Grail to force the system to spawn some bizarre wraith containers based on fictional characters, and Zelretch activating the system to see if it blends.
Granted, the Throne of Heroes exists closer to the Root, than the World, meaning some strange quasi-true magic bullshit may be involved in its operation, but the closest to a "fictional Servant" that exists is Nursery Rhyme, and at least that has several justifications. Or Sasaki Kojiro, but, again, justifications. At least lots of people know about Sasaki Kojiro.
Ado Edem is a member of the Ether Liner race (basically genetically-enhanced humans juiced up on semi-magical space grain), which features prominently in Angel Notes. It describes a hypothetical scenario where Gaia dies and, with its last breath, calls down all of the Ultimate Ones to destroy humanity as a goodbye middle finger. It's a very interesting read.
So, where's Hyrule? North of the Euphrates river?
The idea of "unknown," and "Heroic Spirit" are, perhaps not opposite, but close enough. Whatever these Servants are, they probably aren't Heroic Spirits, and most likely aren't from the Throne of Heroes.