Medea x kotomine
He gets off on suffering, she's got it in spades.
K: Tell me your story.
M: *misery here*
K: Do you want it rough or gentle? *rough anyway*
And, when kotomine isn't infodumping, he's a pretty srs strong and silent type.
And I could totally see kotomine stringing Medea along to keep a devoted Servant.