Servants who Never Should be Summoned.

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Assassin.

He chuckled perversely. "Ah, so I live again! Perfect, perfect. I'm Servant Assassin, the man able to avoid any detection. For there is no greater power than that of--"

The old, decrepit, senile magus stared down at his summoning circle. "Ahhhhh, it failed after all. But I really thought it would work...!"

"No, no, I'm right here!" his Servant said. "It's just you can't see me, because I am--"

He threw his wrinkled, bony hands around, desperately. "My one chance to reach the Root, squandered! All these years of studies, wasted! Why to live, indeed...!"

"PAY ATTENTION!" Assassin yelled at him. "Your summon worked perfectly, fool! I will win this Grail War without nobody being the wiser!"

The old man began to sob to himself. "They'll laugh at me at the academy! There were right, weren't they? Their mockery will chase me to my grave!"

"I'M TELLING YOU I'M RIGHT HERE!" the Servant screamed into his ear. "My name is Hawley Griffin! Are you fucking deaf, damn you?!"

"I can hear them clearly already, calling me a damned deaf fool...!" the Master lamented.

"Oh, for the love of--!"

It took Assassin two full days to get the also extremely myopic man to read a note he'd written for him and make him realize he actually had summoned him after all. Then it only took him a single day to get himself raped and killed by Mr. Hyde again, because some things never change.

The old magus, unaware he didn't have a Servant anymore, still defeated the rest of the Servants on his own and claimed the Grail thanks to assorted hilarious Mr. Magoo-esque hijinks.

The Grail never got to cash on his wish and flood the world in darkness, however, since he just kept wishing things on his teapot instead to the day of his death.