'Rival' (Japanese: ライバル,Raibaru) 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 have a type that is the opposite of the player Pokémon, an example: The player chooses a grass-type Pokémon, then the player rival will pick up a fire-type Pokémon. In exception of Dawn or Lucas, especially Bianca, who all choose the Pokémon with a type disadvantage. Wally starts out with a Ralts, rather than a conventional Hoenn starter. Also, the owner of a certain game can give a rival a name mostly at the start of the video game or later in the story line. The names of the rivals that cannot be named are: Wally, Brendan, May, Cheren, Bianca, Shauna, Serena, Tierno and Trevor.
In the game
Having a rival as the player's opponent has been introduced since Generation I with the known and popular rivalry between Blue/GreenJP and Red. A rival mostly appears in the main game series throughout the game and challenging the player in different Pokémon battles. Rivals mostly appears early in main game series, mostly been picking their starter Pokémon from a Professor in a different region. Their starter Pokémon is mostly the opposite type that the player has chosen, as example: The player choose Charmander, the rival will choose Squirtle, because water is stronger than Fire, but in exception of Bianca which chooses a weaker type than the starter. Most of the players rivals doesn't came to pick up a starter Pokémon, or have seen that they took a starter Pokémon alongside with the player. Silver as an example, stole the Pokémon from Professor Elm's laboratory. Mostly a rival has a deeper story line about their behaviour, most of the player rivals depends more on strength than strategy.
'Pseudo-rival' (Japanese: 擬似ライバル,Giji Raibaru) is mostly one of the main protagonists from the main Pokémon video game series. They are more known as a helping hand in bringing you useful information, giving you useful items and helping you during tag battles. They are seen as rivals in being more in a competitive manner in completing the Pokédex instead depending strength through Pokémon battles.
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 travellings, but his major rivals were: Gary Oak, Morrison, Paul, Barry, Trip and Bianca. Ash have also a lot of minor rivals in the anime series which they are mostly more in collecting badges and holding friendly competitions which some of them achieved in battling with each other in the Pokémon League.
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.