In all games of the main Pokémon series, the protagonist has to deal with at least one rival. While more and more opponents have appeared over the years, most fans remember the rival from Red, Blue and Yellow in particular: Gary Oak.
The first generation includes countless realities, which from then on couldn't be missing in any version. The introduction of the rival, that is, another trainer who sets out on a journey parallel to the protagonist to be crowned the champ of the respective generation, is one of these phenomena.
Gary was initially named Blue in the Red and Blue Versions. In reference to the successful anime series, game developer Game Freak renamed the rival in the Yellow Version: From now on, the protagonist had to deal with Gary.
Professor Oak's grandson is a constant adversary of the protagonist, especially in the first generation. The rival battles in Red, Blue and Yellow sometimes represent the most difficult duels in the games.
While all the rivals from the later generations fail in their quest to gain the title of champ, Gary is initially successful. Thus, after the Elite Four, the professor's grandson stands in the protagonist's way as the last major challenge.
In the second generation (Silver, Gold and Crystal), Gary appears as the Gym Leader of Viridian City. Although the legendary rival awards the Earth Badge, unlike his predecessor Giovanni, he does not rely exclusively on Ground Pokémon. Rather, Gary is the only Gym Leader who does not specialize in any type.
Even though many other rivals were added in the following generations, Gary always remained the only competitor who made it to champ. Even in later games of the main series you can meet him as a protagonist - probably the most legendary rival in the entire saga.