The most fascinating unremarked parallel in the Fate series, to me, is that of Irisviel and Shirou, and particularly that of Irisviel and Archer.
Two beings raised from oblivion by Kiritsugu Emiya, only to become his anchor to humanity and hope for the future. Two selfless nothings made human by Kiritsugu, but left incomplete in the process. Two people who subsumed their will and life to his dream, and ultimately sacrificed themselves on its altar, without truly fully understanding or agreeing with it. But one found herself in the process, and one broke himself in the process.
And what do we make of the strange connection between them that exists in Ilya, the embodiment of familial love in which Irisviel decided she was made whole, and the chance for family that Shirou (and particularly Archer) grasped but could not hold onto? Maybe nothing, except that we can wonder at the way the story of the Emiya family shifts down the generations as it follows incomplete people seeking a way to live.