Partner Pokémon (Japanese: 相棒ポケモンPartner Pokémon) refers to the special Pikachu or Eevee the player starts with in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! As the names of the two titles imply, players of Let's Go, Pikachu! receive a Pikachu as their partner, while players of Let's Go, Eevee! receive Eevee. While the player can capture and train other Pikachu and Eevee in both games, the partner Pokémon have various special differences from their typical variants.
A female Partner Eevee, seen with her unique tail pattern
The partner Pikachu are constantly seen riding on the player's shoulder while Eevee are constantly seen riding on top of the player's head, similar to Ash's Pikachu in the anime, even when not in the active party.
They can also wag their tail to indicate that a hidden item is located in front of the player.
Female partner Eevee have a special heart-shaped pattern around the tip of their tail, similar to a gender difference.
The partner Pokémon have higher base stats, and are fixed to join the party with a fixed all-perfect 31 IVs in every stat, but cannot be Shiny. Their gender can be also determined by the title screen during the start of the game.
They have special new moves that they can learn via Move Tutor, and can activate partner powers in battle once they have high enough friendship. If activated while they are in battle, they use an exclusive move—Pika Papow or Veevee Volley—which increases in damage based on friendship. If activated while they are not in battle, they boost the stats of the current Pokémon.
The partner Pokémon can perform Secret Techniques that take the place of HMs. These Techniques do not take up a slot in the move set, nor are they moves intended for battle.
The player's partner can be interacted with and fed, similar to Pokémon-Amie and Pokémon Refresh in previous games, which can be done by pressing "Play with <Partner's Nickname>" on the menu.
They can also be dressed in different outfits, including sets made to match the player, and be given different hairstyles when pet in certain ways in the Switch's handheld mode.
Though precursors of this feature were first seen in the original series with Ritchie's Sparky, Pikachu and Eevee with different hairstyles as identifying elements began appearing in the Sun & Moon series around the time of Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!'s release. These hairstyles are generally more varied and may not be ones attainable in the games. The first Pikachu with such a hairstyle appeared in Securing the Future!, under the ownership of an AlolanTrainer. In A Plethora of Pikachu!, several other similar Pikachu appeared under the ownership of Pikala.
Both the Partner Pikachu and the Partner Eevee appear as Spirits. Starting in version 1.2.0 of the game, players of Let's Go, Pikachu will receive the Partner Pikachu spirit upon booting the game, while players of Let's Go, Eevee will receive the Partner Eevee spirit. Both spirits can also be found on the Spirit Board. Both spirits are neutral-aligned ace (3★) spirits. Pikachu grants a boost to the user's attack and speed at 0% damage, while Eevee grants an significant temporary stat boost after eating food. Unlike most Pokémon series spirits, Partner Pikachu and Eevee use their boxart artwork from their titular games.