Luka's great-grandfather was a character appearing in Pokémon: Master Quest.
Season 5: Master Quest
Luka's great-grandfather was a sailor, who found out that a ship was transporting goods and contained a Silver Wing. However, the boat crashed and sunk to the bottom of the sea, while nobody believed his story. However, his great-granddaughter, aided by the heroes' help, managed to get the Silver Wing from the ship.