How do you get mime jr as a partner Pokemon on ranger shadows of almia?

You cannot get Mime Jr. as a partner Pokémon in Pokémon Ranger: Shadows of Almia. Mime Jr. is not featured in the game.

The available partner Pokémon in Shadows of Almia are:

* Starter: Mankey, Starly, Buneary

* Later Partners: Riolu, Gible, Shinx, Piplup, Buizel, and others.

You can find more information about partner Pokémon in the game's official guide or online wikis.