cu chulainn and archer emiya consistently bring out the best in each other both directly and indirectly and that is why in both grail wars where only one of them was summoned thew flew completely off the rails and became alter, in this essay i will
for all means and purposes cu and archer are one unit by now because of their cosmic connection, a connection caused by gae bolg having pierced shirou’s heart without killing him way back at the start of fsn. gae bolg is a deeply cursed weapon that has the death of the target as a premise of its use, a curse which even its wielder cannot escape. it is such a powerful conceptual rule that if the target cannot die then gae bolg cannot be used (as seen in scathach’s first interlude) because its use demands that the target dies before it is used due to the reversal of causality involved. shirou’s survival despite the fundamental rule of “heart pierced by gae bolg” == “victim dies” caused a logical paradox that will pit them against each other in as many grail wars as it takes until the day “shirou emiya” in whatever form finally dies by means of gae bolg. cu chulainn and archer emiya conceptually cannot be separated until this requirement is met and the paradox is fixed.
the appearance of cu alter and emiya alter is a direct consequence of them being summoned without the other because alters, as “a side of the servant that normally would never see the light”, are in themselves minor logical paradoxes. the only cu chulainn or archer emiya that can be summoned without their counterpart being present is one that isn’t supposed to be exist at all. their alteration furthermore attempts to compensate for the missing part of the duo by imposing traits of the one who is missing on the other. cu chulainn gained the trait “no heroic pride” in order to become cu chulainn alter, and emiya alter gained the trait “does not resist his fate”, both of which are the main trait which differentiates them from the other and caused their initial animosity towards each other back in ubw route. another way in which this connection can be seen to influence their summoning is in the fuyuki singularity, where a cu without gae bolg is summoned and it causes an equally ‘incomplete’ archer to appear in the same war
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