Proteus Ridley

Proteus Ridley is the unexpected final boss of Metroid: Samus Returns. He is the final incarnation of Ridley's cybernetic Meta Ridley form.

Background
Following his defeats as Meta and Omega Ridley in Metroid Prime 3: Corruption, Ridley seemingly managed to regenerate most of his original body and removed large portions of his cybernetics.

After the Space Pirates had somehow become aware of Samus's mission to render the Metroid species extinct, Proteus Ridley was dispatched to SR388 to intervene. By the time he arrived however, she had destroyed all but one Metroid which had imprinted onto her following its birth. As if foreshadowing Ridley's appearance, a thunderstorm (accompanied by unsettling music) began as Samus and the baby arrived at the Surface after leaving Area 8. A cave-in blocks off the initial entrance to the main caverns, preventing access to the landing site from the opposite side. Facing her Gunship on the top of a cliff, Samus activates it remotely while the baby curiously approaches it. From a distance above, Proteus Ridley emerges out of the storm clouds and heads straight for the Metroid with the intention of bringing it to the rebuilt Space Pirate fortress on planet Zebes.

At the sight of her nemesis, Samus hurries in his direction. He briefly chases the baby, landing on top of her Gunship, before successfully catching it with his left hand, holding it firmly as he is confronted by Samus.

First phase
Starting off in the air, Ridley clutches the baby in his metal hand as he battles Samus. His attacks include breathing five purple fire balls in an arc, thrusting his tail below him in an attempt to jab Samus, and quickly slamming onto the ground before taking flight again. If Samus is Space Jumping at his height and above, or climbing the wall with the Spider Ball, Ridley will whip his tail in a wide arc directly in front of him and alter the angle of his fire balls. Ridley's attacks are very powerful, with many taking out a full Energy Tank. His weak point is both his chest and mouth area, which Samus can take advantage of using Super Missiles, Plasma Beam shots or the Beam Burst. Regular Missiles are ineffective as they will merely bounce off of him. The Ice Beam can also harm him, but considerably less than her other options. After taking enough damage, Ridley will recoil and release plenty of Aeion Orbs to refill Samus's Gauge. A cutscene triggers in which he attempts to fly away with the Metroid in hand. Samus jumps onto his tail, stalling him as he attempts to shake her off, but she immediately flies onto his chest with her thrusters and pumps Plasma Beam shots into him. He screams out in pain and releases his grip on the baby, prompting Samus to jump towards the Metroid and grab it as she falls away from Ridley. When he attempts to charge at her, she lands a beam shot on his left wing, sending him crashing into the nearby cliffside and is partially buried under rubble. However, Ridley quickly rises up and engages Samus again for the second phase.

Second phase
In the second phase, the baby watches the battle at a distance near Samus's Gunship as a storm intensifies in the background. Ridley will now perform additional attacks, some of which are ground-based. When he is located on the ground, he will shoot individual blasts of fire at Samus' position and swipe with his claws if she is close to him; with the latter attack, Ridley may follow up with a second swipe. He takes flight if Samus jumps over him, from which point he will use the previous aerial attacks along with two new ones.

One of new attacks consists of Ridley charging across the area with his entire body and tail extended to the floor, giving this move a wide range. If Samus is close to a wall, he will slam against it and remain still very briefly, though he will turn around instantly if the wall is sufficiently distant. The second new attack is Ridley swooping in to grab Samus which, if successful, will trigger a short cutscene where he slams her repeatedly onto a wall before tossing her. This sequence causes high amounts of damage to her, but can be negated if Samus has the Lightning Armor active; in fact, the ability can be turned on at any moment during the cutscene, but will not remove any damage inflicted prior to its activation. Two of Ridley's aerial attacks from the 1st phase have different properties, such as the tail thrust which now stabs into the ground and releases a wave of spiked rocks when he pulls it out. His ground pounce no longer has him take flight right after and instead switches him to his ground-based pattern.

Once he has taken more damage, more Aeion Orbs will be released from Ridley. A cutscene will then play where he tries to stab Samus with his tail before shielding himself against her attacks as he flies towards her, getting close enough to grab and throw her at a wall. He then pins her on the floor with his left hand and shoots a fire ball at point-blank range, exhausting her. Before Ridley can continue, the Infant Metroid intervenes to save its "mother" and leeches some of his Life energy, but the latter is quickly shaken off. The baby transfers the absorbed energy to Samus, which restores part of her health (equivalent to 2 full Energy Tanks) before the third phase.

Final phase
Ridley becomes more aggressive, using all of his previous attacks at a faster rate as Samus continues to battle him with assistance from the baby. He gains two additional attacks: on the ground, he will use his hyper claw slash that leaves an afterimage on screen, while on the air he will shoot a focused fire ball directly under him which releases a fireball shockwave on both sides from the point of impact. He may also perform his ground slam multiple times consecutively.

The baby helps Samus by draining more energy from Ridley after she inflicts a certain amount of damage to him. Whenever it does, he will shake off the Metroid and lunge himself at Samus, which she can reflect with her Melee Counter as indicated by the white ring emanating from his mouth. Successfully countering it triggers a cutscene where she blasts Ridley in the chest several times with a weapon of her choice, and then climbs up his body before repeatedly shooting down his throat. This does not finish him off however, as the battle continues until Ridley is wounded enough to allow the baby to leech him again, repeating the earlier process a second and final time. This causes Proteus Ridley's cybernetic implants to short-circuit before he falls to the floor defeated.

Alternatively, Proteus Ridley can be vanquished without Samus resorting to the Melee Counter at all and simply continue to damage him as she normally would after the baby's attacks until his defeat.

Proteus Ridley is one of the most difficult bosses in the game; on Fusion Mode his slashing attack which leaves an afterimage is enough to take three Energy Tanks off of Samus's overall health. If Samus dies at any point during the battle, the player will have to restart it from the beginning.

Aftermath
Samus watches an unconscious Ridley before she looks over to the Infant Metroid, prompting her to leave with it in the direction of her ship. Though he is not seen again afterwards in the game, he either regained conscious offscreen or was recovered by Space Pirates, as a Hornoad is seen after the credits gnawing on Ridley's abandoned mechanical claw, before it is attacked by an X Parasite.

Based on his following appearance in Super Metroid and to some extent the abandoned mechanical claw in the ending, he removes the rest of his cybernetics before attacking the Ceres Space Colony where Samus donated the baby Metroid for research. With his fully-restored organic body, Ridley rampages the station; despite Samus' efforts to stop him, he successfully abducts the baby and brings it to the rebuilt Tourian on planet Zebes.

Metroid: Samus Returns Official Guide

 * Inhabitants & Eco System of SR388 (p. 23)
 * PROTEUS RIDLEY
 * "Samus' greatest nemesis makes a return in this game to put a stop to her Metroid eradication. It was Proteus Ridley, Mother Brain, and the Space Pirates that harnessed Metroids for evil during Samus' first mission, and they clearly found it to their liking. Proteus Ridley’s not about to allow Samus to wipe out such a formidable source of power and an equally devastating weapon—not without a fight."

Trivia

 * In Proteus Ridley's official artwork, his neck is still primarily cybernetic and he also has a cybernetic lower jaw. In the actual game, however, only the back of his neck has any cybernetics.
 * Proteus Ridley's visual appearance reflects the progressive regeneration of his organic body. This process was first noticeable with his Omega Ridley form, where his purple body became more pronounced and was pushing off the implants he gained as Meta Ridley back in the first Metroid Prime.
 * Consequently, Proteus Ridley and his later form in Super Metroid have lost much (in the latter case, all) of the protection the metal implants offered throughout the Metroid Prime series, as he can now be harmed by mere Beam weapons shot directly into his exposed organic body.
 * Some regions of Proteus Ridley's body, such as his left arm, feature metal pieces that were not present in his previous cyborg forms, though the cutscene at the end of Samus Returns implies that the new parts are simply removable armor.
 * Owing to him being a variation of Meta Ridley, several of Proteus Ridley's vocal sound effects are recycled from his Meta form, as well as his mouth and chest weak points.
 * Ridley was not originally present in Metroid II: Return of Samus, of which Samus Returns is a remake. Thus his appearance in the game can be considered a retcon. Proteus Ridley is one of two new bosses in Samus Returns, the other being the Chozo excavation robot Diggernaut. Additionally, Proteus Ridley, along with Diggernaut and Arachnus, are the only bosses in Samus Returns that are not Metroids.
 * The addition of Proteus Ridley's battle against Samus and the baby, in which the Metroid's intervention prevents the Space Pirate commander from achieving victory, results in an inconsistency with Samus's dialogue during the events of Metroid Fusion. She states in the latter game that she owes the baby her life twice over, when in actuality it is now a total of three times (due to the retcon) that the Metroid has saved her. The other two rescues consist of the baby's sacrifice against Mother Brain and its DNA being used to save Samus from the X Parasite's infection. Of course this could be explained as Samus simply believing she could have defeated Proteus Ridley even without the Metroid's help.
 * Samus Returns marks the first time that Ridley has been the final boss of a Metroid game; while Zero Mission featured the Ridley Robot as its final boss, it was merely a robot he built in his likeness.
 * His presence in the game was first hinted at in the Overview Trailer, where Proteus Ridley's theme was heard during the Aeion gameplay portion. A glimpse of his hand was also seen in the September 13, 2017 Nintendo Direct, two days before the game's release, with the narrator also hinting at his identity by saying "Vengeance has a name".
 * The Metroid: Samus Returns Official Guide erroneously claims that Ridley has been known as "Proteus Ridley" since before Samus became a Bounty Hunter.
 * In Greek mythology, Proteus is the name of the god of rivers and oceanic bodies of water, who was known for his adaptability and versatility. Therefore, the Proteus name for Ridley is appropriate on two aspects: the reputation he likely obtained prior to Samus' career as a Bounty Hunter, and Ridley's form in Samus Returns as it reflects his ability to easily recover from his battle wounds.
 * There are several moments in the Proteus Ridley battle that reference several scenes from previous games:
 * The climax of Super Metroid, in which the mutated baby saves Samus by draining Mother Brain and transferring the energy to the former, is referenced when the baby drains Proteus Ridley and heals an injured Samus before he could kill her.
 * The confrontation with Ridley's adolescent clone in Metroid: Other M, in which he pins Samus to the ground until she is saved by Anthony Higgs, is played out in a similar fashion when Proteus Ridley pins down Samus until the baby's intervention.
 * One of his attacks used by Proteus Ridley involves him grabbing Samus and slamming her into a wall, somewhat akin to his grab attack from Other M.
 * The free-fall battle with Meta Ridley in Metroid Prime 3: Corruption, where she mounts his head and fires Power Beam shots down his throat, is referenced whenever Samus successfully Melee Counters Proteus Ridley and shoots directly into his mouth with a weapon of her choice.
 * Meta Ridley's defeat in Metroid Prime. Both Proteus Ridley and his Meta form flail at the end of their battles as a result of their body being overwhelmed with electricity.
 * Samus Returns features the only on-screen occasion of the baby attacking in its infant form; all other instances occur while it is in its Big Metroid form. Additionally, the baby is unable to drain Proteus Ridley's energy entirely, though it should be noted that the Infant had yet to develop the size and strength commonly seen in the Larva Metroid stage. Indeed, Proteus Ridley manages to remove the baby off of him several times using merely his hands.
 * The baby contributing to Proteus Ridley's defeat foreshadows future acts the Metroid indirectly performs that also lead to the defeat of Ridley's future incarnations. In Other M, the clone of the baby grows into a Queen and kills Ridley's clone by absorbing his life energy. In Metroid Fusion, the baby's DNA is infused within Samus to protect her against X Parasites, and is what ultimately helps her prevail against Neo Ridley, an X Parasite-imitation of Ridley.
 * Proteus Ridley's theme is altered for each phase of the battle. The first is a remix that combines notes from Vs. Meta Ridley, the version from the Prime Series, and Nemesis Ridley, the version from Metroid: Other M (heard here). In the second phase, his theme has an edgier tone with a more tense opening similar to Ridley's themes in Metroid: Zero Mission and Super Metroid (heard here). The final phase retains the tension but with a slower tempo, similar to that of Neo-Ridley's theme from Metroid Fusion (heard here).
 * It is unknown why Proteus Ridley releases Aeion upon taking damage between the three phases of his battle, as Aeion only exists on SR388, and Ridley is native to Zebes.