Felix Fray is one of the Elemental Twins. His other twin is Elka Fray.
Elka and Felix aren't actual siblings. They were just raised together and were created together. They were created so they could wed each other when they turned into adults.
Their lives started when Eostre in her full form created them to control nature together but something went wrong so they were made separately. Eostre thought about this and decided that when they became older and they wed their powers shall be combined and they will control nature together instead of separately.