The Zeta Metroid is the sixth natural stage of a Metroid's life cycle. They appear as enemies in Metroid II: Return of Samus and its remake Metroid: Samus Returns. They also have a minor appearance in the Restricted Lab of Metroid Fusion. The Zeta Metroid possesses a very different structure from all prior forms, as it now resembles a reptilian dinosaur. This stage is the beginning of the species' adult phase, and thus requires a lot of firepower to kill. Although the Zeta Metroid no longer flies, it is now highly agile and can climb any surface, including ceilings. The Omega Metroid is the next and final stage for all standard Metroids.
Power Bombs are one of Samus' most powerful weapons. They must be deployed via Morph Ball mode; a few seconds after being deployed, they detonate, destroying most enemies within a surprisingly large radius. In Metroid Fusion, the blast also appears to cause a vacuum that can draw ordinary X Parasites (including ones that came from enemies killed by the blast itself) to the center of the blast. They are also capable of destroying Metroids in Super Metroid and the Prime Series.
Fleet Admiral Castor Dane is the commander of the G.F.S. Olympus. His only appearance is in Metroid Prime 3: Corruption. He is known for his utmost dedication to protecting the Galactic Federation from its enemies. He is stern, aggressive, and has a strong dislike for the Space Pirates; his in-game scan hints at him having been orphaned by the marauding race at an early age, an act he still seeks to avenge.
Samus Aran has flown multiple gunships (and starships) throughout the Metroid series, but they have all been designed for roughly the same purpose and are mostly of the "Hunter" class. None of them have an official name outside of Gunship, Samus's Gunship, Hunter-Class Gunship, or Starship, but non-canonical media has named it as the Cosmo Liner and Hunter IV. Additionally, pre-release versions of Metroid Prime and Metroid Prime 3: Corruption referred to Samus' craft as the Stealth Strike Corvette and the Tallon-Class Strike Gunship respectively.
The Thermal Visor is a visor in Metroid Prime, and it is the only Visor that is exclusive to Metroid Prime (although several aspects of the Thermal Visor carried over to the Dark and X-Ray Visors of Metroid Prime 2: Echoes and Metroid Prime 3: Corruption, respectively). It is acquired in the Research Core in the Phendrana Drifts. The Thermal Visor uses thermography, allowing Samus Aran to locate enemies or objects by their heat signatures. It can be used to see Shadow Pirates and is also useful in dark places, extreme cold, and in poor weather conditions, like those in Phendrana Drifts; though it functions poorly in superheated areas, such as the Magmoor Caverns.
Arachnus is a species native to SR388. At the near-top of the food chain, they are very fearsome predators, but seem to be very rare as Samus Aran only encounters two of them in the Metroid timeline, one of which is apparently an X mimic. The biped is distinguished by its three-fanged maw, segmented eyes, elongated neck, a hard shell on its back, and the large claws on its forearms. It is known to curl into an armored ball when threatened in order to jump towards its target in an attempt to ram it, and will uncurl periodically to swipe with its claws or spit fireballs from its mouth. It is encountered as a boss in Metroid II: Return of Samus, Metroid: Samus Returns and Metroid Fusion.
Save Stations (called Save Points in Metroid II: Return of Samus or occasionally Save Rooms) are a recurring feature in the Metroid series. Save Stations allow Samus to save her progress (sometimes referred to in-game as a "data backup") during missions. Save Stations first debuted in Metroid II: Return of Samus and have appeared in every Metroid game since then, making the original Metroid and Hunters the only games without Save Stations. In the games in which it is present, Samus Aran's gunship also functions as a Save Station that also refills health and ammunition, with the exception of Metroid II, where it only serves to restore health and ammo.
Missiles have appeared in every single game yet in the Metroid series. It is a more powerful weapon than the normal uncharged Beam, dealing 4-5 times as much damage depending on the game. They are usually first acquired upon finding a Missile Tank, though in Metroid II: Return of Samus and Metroid: Other M, Samus starts off her missions with them, and in Metroid Fusion through an "update" from the Galactic Federation.
Mother Brain is a recurring antagonist and boss in the Metroid series. She is a cold and ill-tempered AI created by the Chozo, but turned against them when the Space Pirates and Ridley invaded Zebes. Mother Brain saw potential in the pirates and decided that with them helping, she could bring true order to the universe by "resetting everything back to zero". Mother Brain was seen as the main antagonist in Metroid, Super Metroid, and Metroid: Zero Mission. She is also seen in a flashback during Metroid: Other M, and her consciousness was responsible for the events of that game as well.
The Diggernaut is a rogue Chozo mining robot and boss in Metroid: Samus Returns. It is a new boss, and did not exist in the original Metroid II: Return of Samus. Samus Aran first encounters and awakens it in a subterranean cave, later encountering it near a broken Chozo Statue, fleeing as it chases her. Much later on, she re-encounters the robot and destroys it in a battle.
Gandrayda is a Bounty Hunter of unknown origin who appeared in Metroid Prime 3: Corruption. She possessed metamorphic abilities similar to the inhabitants of Jovia XII. She could assume the form and abilities of most living things, including creatures and even machines differing significantly in size. Along with Ghor, Rundas, and Samus Aran, Gandrayda was asked by Fleet Admiral Dane of the Galactic Federation to help stop the Phazon corruption of the Aurora Units.
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.