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 II: Return of Samus
While exploring the planet, Samus encounters Metroids and destroys them, slowly decreasing the planet's Metroid population. During her mission, she notices the mutations that each creature exhibits: the Metroids grow from small jellyfish-like creatures into large, hovering, lizard-like beasts. After destroying most of the planet's Metroids, Samus encounters and battles the Queen Metroid. Killing it, Samus proceeds to return to her gunship through the planet's tunnels.