Explanation on why Evan is Funtime Freddy and William thought he was Cassidy:
After the happiest day, both Evan and Cassidy separated from the rest of the missing children and ended up in sister location.They ended up possessing Funtime Freddy,during this time William was exposed to Cassidy possessing Freddy but not Evan possessing Freddy. However, sometime later, Cassidy left to repossessed golden Freddy, while Evan stayed until he burned in pizza simulator
Hope that helped enjoy✨ 
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