[quote=LittleWing]
I know this doesn't really prove anything due to the fact that the Holy Grail was corrupted at the time, making any manner of weird shit possible, but what about Sasaki Kojirou (Fake Assassin). He was an Imitation Servant or a "Shade" based on a legend about a person who never actually existed.
[/quote]

He was a Wraith, strictly speaking. As one of the nameless swordsmen that served as an inspiration for Sasaki, he was basically the best that Medea was able to bind, since she wasn't able to conjure a Heroic Spirit to serve as her doorman.