Hyakugojūichi is the first ending theme using in the Japanese animated series. The song is sung by Professor Okido's voice actor Unshō Ishizuka.
The animation is quite basic for the ending as Pikachu spends the entire time bouncing a Poké Ball around. The background features what appears to be grass, sky and clouds.
The song talks about how there are roughly 151 different Pokémon that people can become friends with. People can have dreams, memories, experiences and become companions with all these Pokémon.