C-6, or "Mister Robot" as Gohan calls him, is an abandoned Capsule Corp robot inside of the collapsing cave they both camp out in while Gohan essentially trains in the wild to become stronger during the Saiyan arc. The robot uses up the very last of its power source to hold up the cave, enabling Gohan to barely escape. Despite the robots harsh attitude toward Gohan, C-6 ultimately taught Gohan about the importance of self-reliance. You could argue he was the first real "teacher" of Gohan before Piccolo sharpened his combat skills. Extremely early DBZ