Walking Pokémon return to their former prominence in Pokémon HeartGold and SoulSilver, being able to walk with players throughout the entire game.
Though the fact that the starter Pokémon is the first Pokémon that this is allowed for mimics Pokémon Yellow in a way, unlike Yellow, the starter is not the only one able to follow, and any Pokémon obtained by the player can be in this position provided it is the first in the party.
If a walking Pokémon knows a move that can be used on the field, such as Cut or Headbutt, and the correct Badge has been earned (for HM moves), it will be shown to use the move in the overworld when the player comes upon its respective obstacle. When sent into battle, like Red's Pikachu, it will appear from the side rather than from a Poké Ball. Also, when the walking Pokémon faints, the next Pokémon in the party takes its place as the walking Pokémon.
There are few instances wherein the first Pokémon in the party will not follow players. For example, riding a bicycle will send the Pokémon back to its Poké Ball. This will also cause it to be sent out from its Poké Ball when a battle begins, instead of just appearing from the side. Certain locations, such as Blackthorn Gym and the Global Terminal, also prohibit the walking Pokémon from following players. Large Pokémon, such as Wailord, will be sent back to their Poké Balls as well when entering most buildings, as they are too large.
Much like in other games, it is possible to interact with the Pokémon. Checking the Pokémon will cause a small image of its current mood to pop up, as in Yellow. Unlike in Yellow, however, the mood is restricted to the thought bubble above the Pokémon's head. The walking Pokémon will also sometimes interact with the overworld, such as an event Pichu becoming interested in a wild Pichu from the past in Ilex Forest, or an event Arceus allowing players to obtain a member of the creation trio at level 1. Walking Pokémon will occasionally pick up accessories, depending on the location it found the item. In some cases, the Pokémon will refuse to hand over the item it has picked up, and will remain angry at the player for a while after trying to take it. Certain Pokémon will react in a unique way if spoken to in a certain place. For example, speaking to a Sudowoodo on Route 36 will cause it to imitate a tree. When the player calls Lyra or Ethan, they will give information on which Pokémon will react to the player's current location.
Some Pokémon will react either happily or unhappily depending on the environment they're in. For example, Fire-type Pokémon react unhappily if they are forced to walk in water on a beach. The game then states that the Pokémon doesn't like to get wet feet. The game also states that Fire-type Pokémon will appear unhappy for some reason if they are forced to enter a rainy route. Grass-type Pokémon will react unhappily if they are in icy terrains, such as Ice Path, or hotter places, such as Blackthorn Gym.
Sometimes, the walking Pokémon will find a Shiny Leaf, a very rare item, while they walk on certain routes. When it is found by a Pokémon, it will have no apparent presence other than in that Pokémon's status screen. Once five Shiny Leaves are found, Lyra/Ethan will make a Leaf Crown out of the Shiny Leaves, which adds a star on the player's Trainer card.
The ability for Pokémon from the player's party to follow the player around returned in this title. The player can ride certain larger Pokémon, including Charizard, Arcanine, Onix, Rapidash, Dodrio, and Dragonite. In addition, their partner, PikachuP/EeveeE, can always be seen riding on their shoulder or head regardless of what Pokémon they have selected to follow them. In some cases, certain Pokémon will give distinct reactions to certain objects.