Yes, I believe there's something like that in stories from Asian countries...

[He trails off thoughtfully as Ren said that 'Can't have a detective without a criminal'... He would like to think he can retire, and he had done so in life but was he so reductive as a Servant?]

I suppose I should explain it's a little more literal in our case. Some Heroic Spirits are "Paired" for the lack of a better word - if Karna is summoned, most likely his brother and killer Arjuna would follow - that is the same with us - me and that man.