Pokémon Red and Blue are still fondly remembered to this day, and few fans have forgotten the satisfaction of crushing the player's rival, officially named Blue, during the final battle of the game. Blue might have lost at the end of Pokémon Red and Blue, but he is still a big part of the series and has appeared in several other Pokémon games.
All Pokémon games have rivals that players face repeatedly throughout their journey. The majority of these have been disappointments that felt too friendly to take seriously. Gen 7's Hau and Gen 8's Hop were especially annoying, as they felt the need to constantly interrupt the player's quest with banter. Pokémon Red and Blue's Blue remains the best rival in the series, since he was a total jerk and only showed up for battles. Defeating Blue was always a highlight in the early Pokémon games, especially as many fans wanted to do the same to Gary Oak from the Pokémon anime, who was based on Blue.
Regional Champions from older Pokémon games tend to crop up in later titles for cameo appearances. Blue might have only been the Champion of Kanto for a short period, but his popularity with the fans led to him making more appearances across the run of the series.
Pokémon: What Happened To Blue, The First Rival & Champion
Blue is the grandson of Professor Oak and is one of two trainers to depart from Pallet Town at the start of Pokémon Red and Blue, alongside player character Red. The two of them are aiming to become the Champion of the Kanto League. Red and Blue battle each other throughout the game, but Blue manages to become Champion first. In the Pokémon Generations anime, Blue is shown defeating only the Elite Four before Red arrives, seemingly confirming Blue didn't beat an existing Pokémon Champion and instead won the title after defeating Lance. Shortly after, Red arrives and defeats Blue, becoming the new Champion of Kanto.
Blue is encountered again in Pokémon Gold and Silver, at which point he has taken over Giovanni's role as the Viridian City Gym Leader. To date, Blue is still one of the strongest Gym Leaders in the series. He also appears in the Pokémon: Let's Go games, where he aids the player and their rival in defeating Team Rocket, before ascending to his role as Gym Leader. It's mentioned in passing in Pokémon X and Y that Blue traveled to the Kalos region at some point, but it's unclear what he did there. He is also one of many challengers who can be faced in the World Tournament matches in Pokémon Black 2 and White 2, but these matches aren't confirmed to be canonical.
The last canonical Blue appearance was in Pokémon Sun and Moon. Once the player becomes Champion of Alola, they can travel to the Battle Tree. There, they'll encounter older versions of Red and Blue, who are on vacation together. The player can battle either of them once for free, but they can both be faced as Battle Legends during the Battle Tree challenge. Blue might not be able to defeat Red, but he's still one of the most feared and powerful trainers in the entire Pokémon series.