Shounen Saiyuuki Commander Gokuu

After his pregnant mother and father died in a mysterious plane crash, baby Tamiya Gokuro (Goku) was found sleeping in a large block of lava at the scene. Hailed as a miracle baby, Goku was later raised by his uncle and aunt, and is now in his second year of junior high school. The only thing Goku was wearing when he was discovered is a collar, which he still can't remove... What is the meaning of this collar?