The below Pokémon will join your team without you needing to battle them

Magnemite

After rescuing the Magnemite in the second dungeon, one of the Magnemite will ask to join your Rescue Force. It is at Level 6


Absol

After beating Articuno, Absol will join your team however you need to go through a dungeon with him and keep him alive in it! It is at Level 20


Smeargle

After defeating Kyogre, you will be sent onto a mission to rescue a Smeargle. It is at Level 5


Latios & Latias

After going through the Latios Dungeon and returning to town with Latios, you will be sent into another 25 floor dungeon, at the end of it is Latias. Return to town and you'll get both on your team. Latios is at Level 30, Latias is at Level 28


Gardevoir

The Gardevoir that has been talking to you in your dreams throughout the game is found after a dungeon when you're teamed with the Blue Team's Gengar. It will return to town with you...talk to it and Gardevoir will ask to join your team. It is at Level 5


Celebi

Celebi is found at a massive 99 Floor Dungeon where your Pokémon is temporarily reset to Level 1. At the end of the dungeon you will meet Celebi. Instead of battling it, it will join your team. It is at Level 45


 
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