I just comparing the extra's story of each character and I notice something: It's look like Marisa "passed" the test (no matter how much she didn't wanted it) is because, of all the playable characters, she's the one closest to being an "ordinary human", with flaw and vice, and cowardice in her heart. Yet despise all that, she kept on to challenge against Okina with her bravery and stubborness, and won. (In Spell Card, but apparently it's the heart that count, literally)
That got me thinking : Is the two sisters servant's also similarly challenged like that? Does the requirement then is being a human with a strong and tenacious souls then, even though eventually they're going to be puppets? Just how bad of a taste does she have anyway.