In fanfiction writing, loyalty to the Pokemon game can be demonstrated in multiple ways. Firstly, keep the Pokemon's appearance and behavior as close to the game as possible. For example, a Charmander should be a small, orange lizard - like Pokemon that breathes fire and is eager to grow stronger. Secondly, respect the rarity of certain Pokemon. Don't make Legendary Pokemon too common in your story, as they are extremely rare in the game. And finally, when it comes to trading and catching Pokemon, follow the rules and limitations set by the game.