From Bulbapedia, the community-driven Pokémon encyclopedia.
Partner Pokémon are Pokémon who are allowed to always accompany a Pokémon Ranger and do not leave after using a Poké Assist like other Pokémon. So far there are 19 partner Pokémon, spread between the two Pokémon Ranger games, Pokémon Ranger and Pokémon Ranger: Shadows of Almia.
There are only 2 partner Pokémon in this game, they are Plusle and Minun. The one which the player gets depends on their chosen gender, with males receiving Minun and females receiving Plusle.
The number of possible partner Pokémon was greatly increased, now up to 17, one for each elemental type. The player encounters their first at Nabiki Beach, either Pachirisu, Starly or Munchlax. The two not chosen are obtainable later on in the game. Also, throughout the game, the player can obtain different partner Pokémon by completing certain quests involving the Pokémon. However, Kricketot, Croagunk and Mime Jr. are obtained during the main storyline.
Player are to take only one partner pokemon at a time and the rest remain at the partner farm.