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.

Like with Pikachu in Pokémon Yellow, you have the ability to interact with your Pokémon. If you turn around and face your Pokémon, simply press A and you will get to see the mood of your Pokémon. Whether it's happy with you, apathetic or is in trouble due to low health or a status condition. This will allow you to be able to quickly guage how your Pokémon is doing.

In addition to that, there are various Pokémon that have special interactions, with some boosting happiness, depending upon where you are and if it is the right type or species of Pokémon. Many of these interactions you can be notified of by Lyra/Ethan when you call them in the specific area, but there are a fair few that even they do not know of. Below is a table of the Pokémon, the interactions and the locations for many of them. Some interactions may seem to be repeat

As with when you take your Pokémon out for a stroll in Amity Square in Diamond, Pearl & Platinum, when you are out with your Pokémon in this game, your Pokémon will sometimes find items. These items include Berries and Accessories for your Pokémon. The items that they get are dependant upon their location and are as follows;