The reptilian Space Pirate leader who is Samus's nemesis is Ridley. He is a draconic extraterrestrial and one of the Metroid series' most enduring antagonists.
Ridley became Samus Aran's archenemy after leading the Space Pirate raid that killed her parents on the human colony K-2L. Samus was later raised and trained by the Chozo, eventually becoming a bounty hunter who repeatedly confronted Ridley. In the original Metroid, Ridley serves as one of the Space Pirates' major commanders on Zebes, alongside Kraid, while Mother Brain controls the planet's fortress and defenses.
Ridley has appeared in several forms across the series. The cybernetic Meta Ridley appears in the Metroid Prime games, while later stories use cloning and other technology to explain his repeated returns after apparently fatal defeats. He is also the Space Pirate leader who steals the infant Metroid in Super Metroid, drawing Samus back to Zebes.
A common mix-up is confusing Ridley with Mother Brain, the Space Pirates' biomechanical strategist, or Kraid, another powerful commander and recurring boss.