As Moe said, Jack the Ripper in the Nasuverse was an orphaned girl driven to murder do to her horrible situation and frail mental status.

And as it's been pointed out, if you summon someone, you're going to get the closest to them that actually existed. If you summon Arthur, you get Arturia, because she was King Arthur and she actually existed in the Nasuverse.

If you try to summon Sasaki Kojirou... I don't think you'd get Fake Assassin normally, but under the circumstances in Fate/stay night you would get the nameless figure who inspired his legend.

But a requirement of being a Servant is having existed. They have to be figures that existed in the Nasuverse.