Pikachu is the face of the Pokemon franchise and one of the most popular video game characters of all time, but Pokemon's creators didn't originally intend for the Pokemon to become that popular. While Pikachu didn't have a major role in Pokemon Red and Blue, it quickly grew in popularity after the release of the anime series.
In an interview with Game Informer, Game Freak co-founder Junichi Masuda revealed that it wasn't Game Freak's idea to make Pikachu the central character of the franchise. "That was a decision, or rather a suggestion, from the company that made the anime," Masuda said, referring to OLM, Inc. "They originally came to us with the idea that they wanted to feature Pikachu, because Pikachu at the time was really popular amongst kids in school. It is a hard-to-find Pokémon, so kids knew about it."
Thanks to the popularity of the Pokemon anime, Game Freak developed Pokemon Yellow, which adapted several elements from the series, including Pikachu following around the player during their travels around the Kanto region.
OLM, Inc. also came up with the idea of having the Pokemon say their own names in the anime, a trait that's not often used in the games. "They actually had Pikachu’s voice actress, Ms. Ikue Ōtani, do a take on the voice and they showed us a clip of it and we listened to the sound of it saying its name over and over in a really cute way," Masuda said. "We weren’t really sure about it, but it worked out."
While most Pokemon still use growls or other gutteral noises to communicate in the games, Game Freak eventually switched Pikachu's signature sound to match the anime.
The Pokemon franchise's popularity is partially due to the anime, but it's always interesting to see which parts of the franchise Game Freak came up with, and which parts they adapted from the anime itself.