Piccolo is a Namekian. He is Kami's evil half, if either of them die they will both die. In the very begining of Dragon Ball Z he is an enemy of Goku. However, when Radditz arrives he joins up with Goku and together they kill Radditz. Afterwards he takes Gohan away to train him and he becomes friends with him. Then Piccolo is a good guy from that point on.