From Bulbapedia, the community-driven Pokémon encyclopedia.
Sinnoh first partner Pokemon, otherwise known as Sinnoh starter Pokemon, are Pokemon chosen by Trainers at the start of their journey in Sinnoh. These are traditionally given out by Professor Rowan and are to be used to battle the first wild Pokémon that the Trainer encounters.
In the core series Sinnoh-based games, the player can choose a Fire-, Water-, or Grass-type first partner Pokémon at level 5.
Availability
Sinnoh-based games
In Pokémon Diamond and Pearl and Pokémon Brilliant Diamond and Shining Pearl, when two Starly attack the player and rival at Lake Verity, to which they travel upon hearing reports of a red Gyarados, they will choose one of the following from Professor Rowan's briefcase, which he left at the lake.
The player's friend and rival Barry will choose the first partner Pokémon that has a type advantage against the player's choice, and the NPC who is the other-gender choice (Lucas/Dawn) will have the Pokémon weak to the player's choice, and therefore strong against Barry's. After fighting off the two Starly at the lake and returning the briefcase to Rowan, the professor will allow the player and Barry to keep the Pokémon they used.
In Pokémon Platinum, the player and the rival are stopped by Professor Rowan just as they are about to run through tall grass to get to Sandgem Town to ask the Professor for Pokémon. After some questions about the player and the rival's love for Pokémon, he decides to entrust them with one first partner Pokémon each.
In Pokémon Brilliant Diamond and Shining Pearl, the Sinnoh first partners can also be caught in the Grand Underground after obtaining the National Pokédex.
Other games
In Pokémon Omega Ruby and Alpha Sapphire, after the player enters the Hall of Fame for the second time, Professor Birch can be found being chased by his wife on Route 101 and the player will be allowed to choose one of the Sinnoh first partners.
In Pokémon Ultra Sun and Ultra Moon, the middle stages of the Sinnoh first partners can be caught in the wild via Island Scan on Ula'ula Island.
In Pokémon Legends: Arceus, the Sinnoh first partners' first stages, and more rarely the middle stages, can be caught in certain locations, and the final stages spawn as Alpha Pokémon.
Base stat comparison
A postcard from Professor Rowan, depicting the Sinnoh first partner Pokémon
In the anime
In the main series
During the Sinnoh arc of the main series, Professor Rowan was only seen distributing one Pokémon to a new Trainer, Dawn. During her debut episode, Chimchar and Piplup escaped from his lab, and once Dawn assisted in retrieving them, he presented let her keep Piplup, along with a set of Poké Balls and a Pokédex.
In the manga
Pokémon Adventures
In Pokémon Adventures, Professor Rowan is tasked with distributing the Sinnoh first partner Pokémon to Trainer's. After giving Piplup to Platinum, he also gave her the remaining two Poké Balls to distribute to her body guards Paka and Uji, however due to a mix up, they are given to Diamond and Pearl instead who received Chimchar and Turtwig respectively.
Pokémon Diamond and Pearl Adventure!
In Pokémon Diamond and Pearl Adventure!, whilst being in charge of distributing the Sinnoh first partner Pokémon, Professor Rowan is also tasked with adopting Hareta the protagonist of this manga. When Hareta is sent on his adventure with Professor Rowan's assistant Mitsumi he gives them a Piplup and Chimchar respectively.
