From the Super Mario Wiki
Prince Pine is the prince of Jewelry Land and the son of King Fret. During the events of Yoshi's Safari, Bowser steals the twelve magical jewels that unite Jewelry Land. As a result, the peaceful kingdom split in half, creating a light world and a dark world. Additionally, Bowser ordered his Koopalings to kidnap the prince and his father. King Fret was held in the light world, while Prince Pine was imprisoned in the dark world. After Mario and Yoshi defeat Bowser, Prince Pine is released. With all twelve jewels united, Jewelry Land returns to a state of balance.