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.
Big Bad
The main villains in Season 6 were the Trio - Warren Mears (portrayed by Adam Busch), Jonathan Levinson (portrayed by Danny Strong) and Andrew Wells (portrayed by Tom Lenk) - followed by Willow (known in this phase as Dark Willow), portrayed by Alyson Hannigan. Series creator Joss Whedon stated on the Season 6 DVD that the true Big Bad is life itself; and how, as time goes on, it becomes more and more people's worst enemy.