Rival

Rival (ライバル) is a Trainer Class in the game that battles the player often throughout the main storyline. A rival in the game can be a childhood friend of the player (in the case of Cheren, Bianca, Barry, and Hugh) or a bully (in the case of Blue and Silver). When the player is choosing a starter Pokémon, the rival will usually get the type that is strong against the player's Pokémon. For example, if the player chooses a Grass-type Pokémon, then the player's rival will pick up a Fire-type Pokémon. The exceptions are Dawn, Lucas, Bianca, Shauna and Hau, who all choose the Pokémon with a type disadvantage. Wally starts out with a Ralts, rather than a conventional Hoenn starter.

Also, the player can sometimes give their rival a name. The names of the rivals that cannot be named are: Wally, Brendan, May, Cheren, Bianca, Shauna, Calem, Serena, Tierno, Trevor, Hau, and Gladion.

In the game
Having a rival as the player's opponent was introduced in Generation I with the popular rivalry between Blue/Greenundefined and Red. A rival usually appears in the main game series throughout the game challenging the player to Pokémon battles. Rivals mostly appear early in the main story line. Their starter Pokémon are usually the one that has a type advantage against the Pokémon that the player has chosen, for example: If the player chose Charmander, the rival will choose Squirtle, because Water is strong against Fire. Some exceptions are Bianca, Hau and Shauna who choose the starter that is weak against the player's. Most of the player rivals depend more on strength than strategy.

Semi-Rival
Semi-rival (半ライバル) is a rival who is more than a Pseudo-Rival but less than a Rival. They will battle the player, but much less frequently than a full Rival.

Pseudo-Rival
Pseudo-rival (擬似ライバル) is one of the main protagonists from the main Pokémon video game series. They are seen as a helping hand in bringing useful information, giving useful items and helping during tag battles. They are seen as rivals in being more in a competitive manner in completing the Pokédex instead of participating in Pokémon battles.

Anime
In the anime series, many rivals for the main characters has been appeared. The rivals in the anime can be compared to that in the video games. The main characters also learns a lot from their rivals during their competitions. Known rivalries in the series:
 * Known is that Ash has a lot of rivals during his travels, but his major rivals were: Gary Oak, Paul and Trip — all of which were unfriendly to Ash. Gary was a rival during Kanto and Johto, Paul during Sinnoh and Trip during Unova.
 * Ash also had a lot of minor rivals in the anime series. However, they are mostly more in collecting badges and holding friendly battles, which some of them achieved in competing with each other in the Pokémon League. Prominent minor rivals include Ritchie in Kanto, Harrison in Johto, Morrison and Tyson in Hoenn, along with Barry, Conway, Nando and Tobias in Sinnoh, and Bianca; Stephan; Cameron and Virgil in Unova. Tierno and Trevor in Kalos.
 * Jessie and James both have a rivalry with Butch and Cassidy. Jessie has a major rivalry with Cassidy since childhood, the rivalry between James and Butch can be because of the times that they were assign to capture a red Snorlax.
 * May has a rivalry with her fellow coordinator Drew, Harley and Solidad.
 * Dawn has a rivalry with her fellow coordinators Zoey, Nando, Kenny and Ursula.
 * Iris has a rivalry with Georgia, and Cilan has a rivalry with Burgundy.
 * Serena has a rivalry with Miette,Shauna and Nini.
 * Meowth has a rivalry with Giovanni's Persian, mostly because of the fact that the Persian is Giovanni's favorite (which Meowth dislikes because, according to his stories, he was the former favorite of Giovanni).