Ridley (リドリー Ridorī?) is a high-ranking Space Pirate, the archenemy of Samus Aran, and one of the most common recurring characters in the Metroid series, appearing in most of the games. Sadistic and bloodthirsty, Ridley delights in death and destruction, and was responsible for the massacre of the colony K-2L. Since then, he and Samus have clashed several times, with Samus usually emerging the victor. But Ridley's brutality is matched only by his resilience, and he regularly returns from apparent death to do battle with Samus once more. As such, the Space Pirate leader is fittingly referred to as the Cunning God of Death (狡猾の死神?) in his Zero Mission official art on the Japanese website.

Resembling a skeletal dragon with glowing eyes, the true name of Ridley's species is unknown. Despite his bestial appearance, Ridley is known to be highly intelligent and even shown to be capable of speech in the Magazine Z manga. He was also apparently capable of building a robot in his own likeness. Though Ridley often appears in different forms, it is clarified through scans found in the Metroid Prime series, as well as events portrayed in Metroid: Other M, that every one of his incarnations chronologically seen prior to Other M and Metroid Fusion are indeed the same individual (the exception being the Ridley Robot), despite his numerous defeats. Strangely, some of his prior battles ended with his body seemingly exploding, but in-game lore states he survived these violent defeats until Super Metroid. In most games he appears in, he has been the penultimate or ante-penultimate boss. The manga also revealed that Ridley is extremely adept at survival. Ridley was apparently capable of healing his wounds by eating human flesh, as illustrated in the manga in a part where he claimed that he survived the destruction of his flagship by consuming the bodies and cells of the dead humans in K-2L.

His Other M incarnation appears in the Geothermal Power Plant stage for Dead or Alive: Dimensions as a stage hazard, owing to both games being developed by Team Ninja. Aside from this, he also had an "appearance" on the April Fool's edition of Game Informer in 2007, where it joked that Team Ninja was licensed to do a Metroid game called Metroid Prime Xtreme (referencing the Xtreme series), and showed Ridley playing volleyball in a bikini with various Metroids and Samus.

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