Marie's grandfather is a minor character from Pokémon: Johto League Champions.
Season 4: Johto League Champions
When the heroes wanted to cross the river, they asked the old man for a ride. The old man wanted a Sudowoodo, so the heroes went after it. Team Rocket also wanted to cross (since their balloon was destroyed), so they went after Sudowoodo. When Team Rocket got it, Sudowoodo ran away due to it being scared by water, since James wanted to use a Squirtbottle. In the end, the grandfather was pleased, since Marie and Pierre made their discovery, as he wanted a Sudowoodo so her granddaughter could've examined it. So, he took the heroes across the river.