The Name Rater NPC is located in a house northwest of the Slateport City Pokémon Center. You can access this NPC as soon as you reach Slateport, and he can change the nickname of any of your Pokémon – as long as you’re their Original Trainer (OT).
So any Pokémon you receive through trades can’t have their nicknames changed by the Name Rater.
Location
Head to Slateport City’s Pokémon Center to get started.
Step 1: From Slateport City’s Pokémon Center, follow the path on the left heading north.
Step 2: Then just look for the Name Rater’s house on the left-hand side of the path – it’ll be just below the Contest Hall.
Step 3: Once inside, you’ll find the Name Rater NPC standing next to a table.
Talk to him when you’re ready to rename your Pokémon.
NPC Mechanics
The Name Rater allows you to rename your Pokémon, adding a layer of personalization in your playthrough. You have the freedom to choose cool, cute, or even joke nicknames to your Pokémon – the choice is yours!
Using the Name Rater’s service is pretty straightforward.
Just approach him, select the Pokémon you want to rename, and then input the new nickname.
Keep in mind that nicknames can only be up to 12 characters long.
If you try to rename a Pokémon that wasn’t originally yours, the Name Rater will kindly refuse, saying: “That is a truly impeccable nickname! I can’t say anything bad about it!”