Ridley (Metroid)
Often considered one of the most memorable Nintendo villains, Ridley has been a mainstay of the series, having appeared (through various incarnations) in almost every "Metroid" game to date (all but "", "Metroid Prime Hunters" and ""). He has also appeared in the "Super Smash Bros." series, however, he had not been made a playable character until "Super Smash Bros. Ultimate" in 2018, making it also the first game overall to have Ridley as a playable character.
Metroid (fictional species)
Metroids appear in almost all games in the "Metroid" series. One particular Metroid, the Baby Metroid that imprints on Samus Aran after she destroys the Metroid Queen in "", becomes an important character in later games. They also made an appearance in "Kirby's Dream Land 3", the "Kid Icarus" series (under the name "Komayto"), and in "Super Smash Bros. Brawl" and "Super Smash Bros. for Nintendo 3DS and Wii U".