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This article contains plot details from the latest books.
Cognates are a pair of Telepaths who work together and have a special mental connection. They can achieve things that a single Telepath cannot by combining their energy and balancing each other's personality. Cognates are required to share everything with each other and their bond is based on trust. The special bond between cognates is strong but can be broken if the trust between the two is breached. It is very rare for a Telepath to find their desired match, much less achieve Cognatedom, because they have to be so open and perfect for each other. It can be very hard to share everything with another person, shown by Sophie in Neverseen, Lodestar, and Nightfall.
Note: There is an even higher level of trust; however, this is the highest level that both members are only themselves, and not another as well. For example, Mr. Forkle and his deceased twin were pretty much two bodies with one soul.