He is a being made entirely out of the rocks on the Blue Moon. He is also the only inhabitant of the planet until Turbo Thunder became stuck there and made Rocky.
When the Chosen One arrived at the Blue Moon, the Rock Guardian was to give a test as proof that the person claiming to be the Chosen One was indeed the Chosen One. Turbo Thunder threatened the Rock Guardian and was knocked away by him. However, Timmy Turner refused to fight him; thus proving that Timmy was the Chosen One. He was absorbed by the Darkness after he gave Timmy the Wind Wand. He was later restored after the Darkness was defeated. He has not appeared since Wishology!.