Which Pokémon has always accompanied them on their adventure?

I always look for guides on how to capture Feebas in every game of the franchise that I haven't played. Milotic is fantastic! I have the Milotic from my old Pokémon Ruby game, accompanying me in every game.

I discovered Milotic as a game glitch. I thought Feebas was a bug in my Ruby cartridge, but I decided to experiment to see what would happen if I caught a glitch and how my playthrough would end. Come on, it was the mind of a kid playing scientist with his adventure progress!

I realized it actually was a Pokémon and decided to train her until I read about its evolution method on a forum!