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The three lake guardians represent intellect, wisdom, and will. The three fundamental strengths of humanity simultaneously embodied and taught by the lake spirits. Im not sure what fourth boon mankind could arguably possess. Maybe empathy, if you want to get mushy.
Would make for an interesting scene where Cyrus captures three of the four guardians, only failing to get his hands on the one spirit he cannot comprehend.
(Being an autistic ubermensch and all)