In Pokmon Yellow, a new feature was introduced which had your starter Pokmon, 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 Pokmon. In Heart Gold & Soul Silver, you once again get a partner
Pokmon. However, it is not limited to your starter Pokmon.

In Heart Gold & Soul Silver, as soon as you get your Pokmon, Professor Elm will say
that your Pokmon can come out and walk along with you. Originally, this would seem to
follow the precedent set with Pokmon Yellow, however every single Pokmon 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 Pokmon at the top of your team and it
shall be sent out.

Interaction

Like with Pikachu in Pokmon Yellow, you have the ability to interact with your Pokmon. If
you turn around and face your Pokmon, simply press A and you will get to see the mood of
your Pokmon. 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 Pokmon is
doing.

Items

As with when you take your Pokmon out for a stroll in Amity Square in Diamond, Pearl &
Platinum, when you are out with your Pokmon in this game, your Pokmon will sometimes find
items. These items include Berries and Accessories for your Pokmon. It is unknown if
there's just a set list for the Pokmon to get or if each Pokmon has a unique list of
obtainable accessories.