Why do HeartGold and SoulSilver let a Pokémon follow you?

The following Pokémon began in 1998's Pokémon Yellow, where one Pikachu refused its ball and walked behind the player in tribute to the animated series, its mood checked by turning to face it. For a decade that was the whole feature: one famous mouse, in one game, as a piece of anime fan service.

HeartGold and SoulSilver, remaking the generation that followed Yellow, universalised it. Whichever Pokémon stands first in your party walks behind you in the overworld, all 493 species, from Magikarp flopping along the promenade to Ho-Oh trailing its trainer through town. Turn and talk to it and it reacts to the place and the moment: shivering in cold caves, dancing in pleasant weather, finding accessories to dress up with and, now and then, picking up an item it wants you to have.

The charm is engineered, not incidental. The feature gives a visible body to the partner you are raising between battles, in a pair of games whose whole pitch is companionship, and it pairs with the Pokéwalker in the box: your lead Pokémon walks with you in the game, and another walks with you in the world. Only the largest species bow out in the tightest interiors, returning to their balls where the sprite would not fit the door.

Yellow's Pikachu had been a novelty. The remakes proved the idea's real weight, and the walking partner has kept returning to the series ever since, in remakes and new games alike, always traced back to the Johto pair that let everyone out of the ball.