Friendship (also known as Happiness) is an aspect in the Pokémon games. It is literally the bond between trainer and Pokémon. Friendship can be gained by winning battles, using certain items, and healing the Pokémon. The higher the friendship, the more the Pokémon likes the trainer. All Pokémon can reach high friendship. However, strong Pokémon and Legendary Pokémon are hard to befriend.
In HeartGold and SoulSilver, the aspect of friendship can be viewed by interacting with the Pokémon in front of the party. When friendship is high, Pokémon can show signs of affection towards their respective trainers. Some of these signs can be strong. Friendship can determine the power of Return or Frustration. If the Pokémon's friendship is high, it likes its trainer and the power of Return increases while the power of Frustration decreases. If the Pokémon's friendship is low, it dislikes its trainer and the power Frustration increases while the power of Return decreases. Capturing a Pokémon with a Luxury Ball, attaching a Soothe Bell to the Pokémon, or giving it berries that lowers Stats are ways to increase friendship with your Pokémon. However, letting the Pokémon to Faint in battle and letting it to get injured a lot will decrease its friendship to the trainer, making the Return attack weaker but making Frustration stronger.