Mecha Guardians

This is a world where aliens have taken over tye world except hidden domes where humans have managed to survive inside but risk lives to get food and when everyone turns 13 they're taken away and chained to be Mecha Guardians and Mecha's are legendary High Tech robots from space and if a human bonds with a Mecha the Mecha can unleash it's true power but Mecha's can't be created but they can be destroyed and repaired. Also there's a chance a Mecha doesn't like someone and can pick a different Guardian as they call it. You are a 16 year old (can be M or F) and have passed be a cadet and you were chosen by the one Mecha who never had made a bond with a human and the Mecha is named Striker