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 (although strong Pokémon and Legendary Pokémon are hard to befriend). Also, some say that if a high-level Pokémon has the highest friendship level with a trainer and it was another's Pokémon, it will listen to you. This is not the case if you don't have the respective number of badges; but even if the Pokémon is friendly towards you, they still will not listen to you.
In the Games
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 augment Stats are also ways to increase friendship with Pokémon. However, allowing the Pokémon to faint in battle and letting it get injured a lot will decrease its friendship to the trainer, making the Return attack weaker, but making Frustration stronger.
Friendship can also help certain Pokémon evolve, such as Togepi into Togetic and Golbat into Crobat. Many Baby Pokémon evolve into their base Pokémon though Friendship. Additionally, in Diamond, Pearl, and Pokémon Platinum, one can check their Pokémon's friendship level with a Pokétch app. Rattata are jerks though so dont even bother with them they hate you no matter what.
Friendship Evolution Pokémon
|List of Pokémon Evolves by Friendship|
|Alolan Meowth||Alolan Persian|
In the Anime
In the case of Greninja, his bond with Ash was so strong that he is able to activate Ash-Greninja mode which may be able to surpass even the Mega Evolution.