They're kind of like the other side of the coin as Heroic Spirits and act as a balancing counter-force of humanity itself whenever there's a factor for the extinction of mankind.
Instead of being the spirits of those who became legendary though great feats and become elemental spirits of humanity, counter guardians are unknowns who achieved a single great feat through some outside force. What they typically do is make a contract directly with the World in exchange for a miracle during their lifetime.
Supposedly after Saber returns to her own time and dies, she would become a Counter Guardian just as Archer did, since she made a similar contract. OTOH, she's...well, pretty famous for her reign outside of that, so it looks like she could go either way.
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Instead of being the spirits of those who became legendary though great feats and become elemental spirits of humanity, counter guardians are unknowns who achieved a single great feat through some outside force. What they typically do is make a contract directly with the World in exchange for a miracle during their lifetime.
Supposedly after Saber returns to her own time and dies, she would become a Counter Guardian just as Archer did, since she made a similar contract. OTOH, she's...well, pretty famous for her reign outside of that, so it looks like she could go either way.