Disclaimer:Spoilers for the ending of the Secret Episode of Re:Mind, if you haven't gotten it for yourself or watched it online and don't want spoilers please don't read this
As a Homunculus and a Grail Vessel Irisviel Von Einzbern should not have been capable of summoning a Servant, so why is it that on the night that Kiritsugu is to summon King Arthur a young boy with eyes like the sky is bound to her?
After his climactic battle with Yozora ends with Sora just barely emerging victorious and being unceremoniously dumped back into The Final World, he expects to spend the rest of eternity watching over the Hearts that exist there, only for him to suddenly here the cry of a faraway Heart begging for the safety of her love.
Of course, being the person that he is, he can't exactly ignore what is *obviously* a call for help, so he traces their newfound connection back to her in order to help her keep her love safe.
This whole "Holy Grail War" business is a little bit beyond his understanding, but that's ok because if there's one thing Sora is good at it's getting involved in stuff he didn't understand and saving the people that needed to be saved, regardless of how long it would take him.
Irisviel and Kiritsugu deserved better, change my mind