In Pokémon Yellow, a new feature was introduced which had your starter Pokémon, Pikachu, follow you around. Pikachu was always behind you and you could always check it to see its happiness. This feature later returned in Amity Square in Diamond, Pearl & Platinum, with a limit to 20 Pokémon. In Heart Gold & Soul Silver, you once again get a partner Pokémon. However, it is not limited to your starter Pokémon. In Heart Gold & Soul Silver, as soon as you get your Pokémon, Professor Elm will say that your Pokémon can come out and walk along with you. Originally, this would seem to follow the precedent set with Pokémon Yellow, however every single Pokémon can be sent out to follow you. From Bulbasaur to Arceus, you can have any of your favourites follow you. To do this, all it seems you need to do is have the Pokémon at the top of your team and it shall be sent out.