The codes below will insert a special Trainer into your game against who you can battle in the house with the locked room at Mossdeep City (south-east of the Pokemon Center).
All the codes are to be used on the Action Replay v3/4
English Ruby/Sapphire (any version, v1.0/v1.1/v1.2)
These codes are also working on other European and American language versions (English/French/German/Italian/Spanish) but introduction as well as win and lose phrases might be grammaticaly incorrect for some Trainers in non-English games.
Japanese Ruby/Sapphire
Remove Trainer data
Ruby/Sapphire (ENG)
2210F546 4E758A05
3A8313FA 554F4427
Ruby/Sapphire (JAP)
A63C0E19 C80ADDD1
3A8313FA 554F4427
Master Codes
I won't port any of the above codes to Emerald or FireRed/LeafGreen.
They won't work properly since Trainer sprites have different indexes in Emerald and in EU/US versions the Trainer names also have the same restrictions as in Japanese versions (only 5 characters allowed).
If you want to port these codes or make your own feel free to do so!
Trainer data structure
Documented by Adam from Furlock's Forest
Trainer Classes: RS, FRLG, E (couples and special NPCs like Team Aqua/Leaders/Rivals don't have their proper overworld sprite)
RAM offsets