Piron no Himitsu 

Brothers Tetsuo and Akio, who were camping in the mountains, meet a mysterious boy named Piron. Piron is actually the prince of the star Castor in the Gemini constellation. His father was displaced from power by minister Valles, and Piron escaped to Earth with Mira, his robot companion. Tetsuo and Akio, along with Mira, decide to help and protect Piron from Valles' assassins, who are sent to Earth after him.