Turtwig is the first Sinnoh starter Pokémon to join Ash’s team in Pokémon the Series: Diamond and Pearl.
Ash catches Turtwig early in his Sinnoh journey after encountering it in the wild. Turtwig is independent and protective, but it gradually accepts Ash as its Trainer and becomes a dependable member of his team.
Turtwig later evolves into Grotle and then Torterra. Its final evolution is powerful, although Torterra’s increased size and slower movement create a different battle style from the quick Turtwig seen at the beginning.
Chimchar also joins Ash during the Sinnoh series, after previously belonging to Paul. Piplup is Dawn’s partner, while Gligar is another Pokémon Ash obtains later. The question concerns the order in which Ash’s Sinnoh starter Pokémon join his team, not the order of the region’s game choices.