Ninja

Ninja (or "shinobi" in historical Japanese) are Japanese covert assassins often for hire or belonging to a shady clan. They tend to specialize especially in stealth-oriented affairs such as sabotage or infiltration although they are able to competently fight head-to-head. Their counterparts are the battle-oriented 'samurai'.

Ninjas in the Mortal Kombat series
Ninja is a loosely used term in the actual sense of ninjas regarding the Mortal Kombat franchise, thus they are a very prominent group, featuring 25 characters listed under this classification. It is used as a classification to those who wear a colored uniform and mask to conceal their face. Common attire is a broad vest that splits apart at the chest area in a V-formation as well as boots or 'tabi'. A short cloth is tapered at the waist area. The attire is completed by a mask to conceal their identity though some of the characters have often removed their masks for deadly purposes. It is not uncommon for a ninja to possess supernatural powers that gives them the edge over their opponents. From Mortal Kombat to Mortal Kombat Trilogy, ninjas were virtually identical to one another in terms of appearance, save for their color choices and powers. From Mortal Kombat 4 onwards, ninjas were given a better sense of diversity with their designs.

Canon

 * Scorpion - Perhaps the only character in the Mortal Kombat Universe who can actually be considered an actual ninja, Scorpion is the only known member of a Japanese ninja clan, the Shirai Ryu. Formerly known as Hanzo "Sasori" Hasashi, Scorpion is a spectre who rises from the dead to avenge the demise of his family and clan. At first, he had thought Sub-Zero to be the perpetrator until Quan Chi revealed himself to be his family's true killer. Scorpion has since made it his goal to kill Quan Chi, no matter what. Scorpion dresses in yellow shades with a skeletal theme reminiscent of his undead status (e.g lower mask looking like a ribcage in Mortal Kombat vs. DC Universe). He wields a spear weapon which he uses to fling at his opponent and reel him or her in.
 * Noob Saibot - Formerly known as Bi-Han. The first man under the moniker of Sub-Zero and the current one's older brother. Killed by Scorpion, Sub-Zero plummeted into the Netherrealm and became a wraith with no recollection of his past morals as a human. Noob Saibot aims to further his goals of conquest with the help of Smoke's nano-technology to create an army of cyber-demons that will follow his command. He appeared, initially, as a solid black silhouette, and later adorned belts and a tight-fitting suit to fit in with his new-found assassin theme. He possesses the ability to meld with the shadows and other phantom-themed powers.
 * Reptile - Shang Tsung's former bodyguard one of the two lone survivors of the near-extinct Saurian race. He is constantly seeking a worthy master to whom he will serve, showing the utmost loyalty to them. He dresses in green and black garbs, though because of his separation from his clan, he begins to appear less and less like a human and more like a beast, eventually no longer wearing a mask. He possesses abilities inspired by several reptilian creatures.
 * Sub-Zero - Real name, Kuai Liang. As the successor to his older brother, Sub-Zero is a member, and later, the grandmaster of the Lin Kuei clan, an organization of Chinese warriors often mistaken as ninjas due to their costumes. Like most clan members, Sub-Zero dresses in blue. From Mortal Kombat 3 onwards, he bears a scar over his right eye. Starting in Mortal Kombat: Deadly Alliance, he wears the Dragon Medallion, an item of great power. And from Mortal Kombat: Deception onwards, he wears armored plating adorned with dragon markings. True to his moniker, he has a master over the element of ice, being descendant from Cryomancers, using it in a variety of defensive and offensive ways.
 * Smoke - Real name, Tomas Vrbada. A former Lin Kuei warrior from the Czech Republic who fell victim to, but overcame subjection of, the roboticization process. Sub-Zero's closest friend and ally, Smoke has been reprogrammed by his older brother, Noob Saibot for his own ends. He can do naught but obey the evil wraith's commands. He wears silver and black armor. Due to the recent reboot, Smoke retains his human form. While his update retains some elements of his human ninja form, he is now depicted without a hood, revealing long white hair swept up to invoke a wispy theme. Smoke possesses incredible speed and the ability to manipulate his intangible namesake for a variety of purposes as well as being able to morph into a cloud of smoke.
 * Ermac - A ghastly legion of warrior souls fused into one single entity, referring to themselves in the plural form instead of singular. Formerly a pawn of Shao Kahn, Kenshi freed Ermac allowing him to join the side of good to atone for what he had done in the name of evil. Ermac dresses in reddish-orange with a certain warrior theme, and wears a mask completely composed of black, leather ribbon. Has the power of telekinesis and a few soul related powers.
 * Rain - A self-proclaimed prince of Edenia and a son of Argus. An orphan who bore witness to the invasion of his home by Shao Kahn's Outworld forces. Once a minion of the ruthless emperor and a willingly traitor to his own realm, Rain fights for evil. He dresses in purple garbs with a royal theme. He possesses the ability to control the weather thanks to his half-god heritage, using water and lightning to drown his opponents.
 * Chameleon - An enigmatic warrior shrouded in mystery. His past and ambitions are unknown to all. Wears a quilted garb that changes colors periodically and has transparent skin. He possesses powers based on other male ninjas.
 * Tremor - A member of the Black Dragon, possible former member of the Lin Kuei, he was defeated by Jax. He has the ability to create shockwaves in the ground much like Jax himself, and the ability to create 3D fireballs.
 * Wu Lae - the leader of the Tengu clan located in Arctika. He uses a scythe in battle. His clan is a rival to the nearby Lin Kuei, but the two share an uneasy truce in which the Tengu remain out of the Lin Kuei's territory.

Non canon

 * Classic Sub-Zero - Classic Sub-Zero was a playable character in Ultimate Mortal Kombat 3 and Mortal Kombat Trilogy, initially believed to be the live representation of the deceased original Sub-Zero. His biography screen is almost identical to the younger Sub-Zero's in Mortal Kombat II; it states that, though believed killed in the first Mortal Kombat, he is thought to have returned to try and assassinate Shang Tsung.
 * Hydro - He was a younger but close friend of Sub-Zero and journeyed with him to combat the forces of Outworld. His unique talents allowed him to manipulate water. His control over the element allowed him to blast strong waves of water at his enemies. Hydro was killed by Scorpion in Outworld as part of the undead wraith's promise of killing all of Sub-Zero's friends and family before killing Sub-Zero himself. Like Sub-Zero, he wore shades of blue but had a unique variation of the traditional Lin Kuei garb which excluded the black shades.
 * Komodai - Komodai was a male Saurian that invaded Earthrealm. It is not known where he originated from except that he was accompanied by a team of fellow Saurians that broke through a dimensional portal into Earthrealm where they fought the "Defenders of the Realm."
 * Oniro - Oniro was a male human and the Grandmaster of the Lin Kuei before its reformation. It's not known how long Oniro held the title of Grandmaster except that he was present before the order was given to automate their ninja assassins. He was present when both Cyrax and Sektor went through the cybernetic process at which point both Sub-Zero and Smoke refused to take part in. He dresses in white and black robes. but leaves his face exposed. Possesses the ability to shapeshift partially or wholly into dark-colored animals.
 * Yong Park - Yong Park was a former Lin Kuei ninja (the Lin Kuei are described as practicing the art of ninjitsu in the novel) turned toll-taker. He was killed by Sub-Zero twenty years later for leaving the clan, a traitorous act punishable by death. His soul merged with the body of his son Tsui Park to become Scorpion.

Cyber ninjas
It has never been clear as to how mechanical the automatons truly are. In Mortal Kombat 3, a damaged cyborg sparks and the skeleton is revealed. The skeleton is almost completely mechanical, aside from the rib cage and skull. The rest of the body is filled with odd mechanical parts and wires. This was about as complex as the cyborgs were at the time. In Mortal Kombat Gold, it was revealed that the cyborgs wore helmets and retained their faces underneath; Cyrax's alternate outfit and ending revealed this although this seems to have been somewhat retconned with the Klassic costumes for MK 2011 where the head and brain are the only things left.
 * Sektor (Unit LK-9T9) - Sektor was the first to undergo this process of roboticization, and he would prove to be their biggest success. The Lin Kuei programmed the cyborgs to do their bidding, and Sektor's main objective was to seek out and kill Sub-Zero, who left the clan after refusing to be transformed into a machine. Sektor was devoid of a soul, lost all human emotions, and has never succeeded in his mission. He later founded the Tekunin, a group of cybernetic ninjas. He has a red-colored sheen with a more battle-oriented theme and is armed with various missile weapons and a portable wrist flamethrower.
 * Cyrax (Unit LK-4D4) - Cyrax was the second Lin Kuei warrior who was turned into a cyborg. However, Cyrax was captured by Sub-Zero, reprogrammed, and permanently malfunctioned. He was then captured by Sonya Blade and Jax Briggs, and was partially returned to his human state. In gratitude, Cyrax joined the Special Forces. He has a yellow sheen and is armed with time-delay bombs and an energy net.
 * Smoke (Unit LK-7T2) - Smoke was the third member of the clan to transform into a cyborg. After returning from a mission, he and Sub-Zero learned of the Lin Kuei's plans to transform their members into robots. The two fled; Sub-Zero escaped, but Smoke was captured and forced to become a cyborg. He was later captured in Outworld by Shao Kahn's army. Hopelessly imprisoned, he chose to shut himself down. Afterwards, Smoke was found by Noob Saibot and reactivated, along with some tampering of his machinery. The two would form a team called Noob-Smoke. As of now, Smoke is torn between serving his new master and searching for his lost humanity.
 * Cyber Sub-Zero (Unit LK-52O) - Due to the changing of timelines by Raiden, Kuai Liang is captured and automated instead of Smoke. Though he temporarily served Shao Kahn through the slaving protocols installed in each cyborg by the Lin Kuei, they were disengaged by Jax after being defeated by Kabal and being brought back to Raiden and his allies, giving Sub-Zero back his free will and again fought for Earthrealm. He keeps his blue coloration and retains his ice powers as usual but appears to be more powerful than the two elite cyborg ninjas Cyrax and Sektor.

Female ninjas
Mortal Kombat II had marked the debut of the kunoichis (in historical Japanese), female counterparts of male ninjas. Unlike their opposite sex, their basic designs change drastically throughout each game. They were first depicted with a leotard, a plain mask, head band and tight skin boots. By Ultimate Mortal Kombat 3, the design changes to a more detailed leotard split in the middle held together by thick threads, a mask, detailed boots and hair tied up in a small bun. As far as abilities go, they are just as lethal in combat as male ninjas, prominently employing the use of weapons in favor of hand-to-hand combat. Their slim, lithe build, combined with their agility and speed makes them much more difficult to handle in kombat.

Canon

 * Kitana - The 10,000 year old princess of Edenia. Kitana was adopted by Shao Kahn as his stepdaughter after the invasion of her homeworld and the death of her parents, King Jerrod and Queen Sindel. She eventually rebels against the tyrannical emperor and joins Earth's forces in defeating the Outworld menace. Having rescued the resurrected, but brainwashed, Sindel from Shao Kahn's control, Kitana rules her liberated realm alongside her mother until the events of Mortal Kombat: Deadly Alliance where she was killed. Kitana is then revived under Onaga's control and then set free by Liu Kang and Ermac. Wears royal-blue colored shades, fitting in with her status. She possesses several mystical powers such as flying and teleporting and wields a pair of steel fans in kombat.
 * Mileena - The half-Edenian, half-Tarkatan clone of Kitana, created by Shang Tsung to serve Shao Kahn, as well as Kitana's 'stepsister'. As time grew by, her ambitions and resentment towards her original counterpart's status and beauty caused her to take up arms against Edenia. Wears a hot pink uniform. Unlike Kitana, Mileena's attire is much more revealing and decidedly Arabian in style, compensating for the hideous visage she hides with her veil. She possesses a couple of bizarre abilities and a pair of sai during kombat.
 * Jade - Kitana's loyal bodyguard and closest friend. Jade survived the invasion of their homeworld but stayed in secret. She was later enlisted into Shao Kahn's army to track down Kitana, dead or alive. Choosing her friendship and loyalty over the emperor, she rebels against Kahn alongside her old friend, taking back Edenia. After the Deadly Alliance's triumph over Earth's forces, Jade and Sindel try to locate their fallen brethren, whom had already fallen under Onaga's spell. She wears shades of her namesake, retaining a variation of the traditional Ultimate Mortal Kombat 3 ninja uniform. She possesses, among other abilities, the power to reject projectiles, and wields a steel bo and 'razorrangs' in kombat.
 * Khameleon - Another mysterious survivor of the Saurian race and searching for Reptile, the only other known survivor of their extinct race. Khameleon almost succeeded but failed when Earthrealm's force triumphed, which had forced Reptile to run into hiding. She later resurfaces during Mortal Kombat: Armageddon in order to exact her revenge towards Shao Kahn, indirectly siding with Earthrealm's forces. Khameleon, like Jade, retains a variation of the Ultimate Mortal Kombat 3 ninja outfit, but wears shades of gray and transparent skin. She possesses the powers of other female ninjas, including their weapons.
 * Tanya - An Edenian traitor, and daughter of an ambassador, she initially allowed Shinnok's entry into Edenia. Has since earned the ire and hatred of Jade. Tanya wears yellow clothes, and has stealth-based moves, such as the ability to turn invisible.
 * Frost - The protege to Sub-Zero, Frost won a competition to join the ranks of the Lin Kuei and took on this position to steal the amulet that heralded leadership of the Clan. She has powers similar to both of the Sub-Zero brothers, to a lesser extent. She often manifests in the ability to freeze the ground near herself or her enemies.
 * Skarlet - A mysterious character that appeared as DLC in Mortal Kombat (2011). She wears red apparel and features long, red hair, her weapon of choice being a pair of knives. She was based on a glitch that appeared in MKII. Composed primarily of warrior blood shed from countless warriors bound together by sorcery, she is Shao Kahn's personal enforcer, released only for the most dire of tasks. Being made out of blood, she utilizes several blood-related techniques, including the ability to morph into a puddle.

Non canon

 * Ruby - Warrior in Ermac's squad in Defenders of the Realm. She was sent by Shao Kahn to invade Earthrealm and sabotage their ability to enter Outworld. She eventually assisted Earthrealm's warriors in defeating Ermac's forces.

Behind the scenes
In reality, all of the ninja characters in the first three MK games were all represented by a single actor. The original base costume was of a red color to avoid blending in with the blue background used when filming the characters for the sprites. Although this method was used to squeeze in more characters while saving memory space, it incidentally gave the actor more work to do than the others. The ninja actor has to perform a wider array of moves and multiple fighting stances than the others as they would be "portraying" multiple characters at once. Once the sprites were captured, they were digitally recolored, creating color values to be applied to the numerous characters. These values were programmed and coded to be treated as separate character values, having access to certain sprites and individual special moves to create their identities.

Because of this, there is a possibility to 'unlock' access to all of these sprites to any ninja, granting them different abilities by swapping movesets or give them every single special move. This would later be explored by the Mortal Kombat Trilogy hidden characters Chameleon and Khameleon.

DLC information
When Mortal Kombat (2011) neared release, DLC information was made available for those who were planning on preordering the game. Depending on the location of the purchase, the user would get a different costume based on the MK1 incarnations for the male ninjas and the MK3 incarnations for the females. Jade however, is an exception as her MK3 costume can be unlocked by purchasing the Blu-ray versions of the Mortal Kombat films.

As of the second Compatibility Pack, two more retro costumes have become available for Smoke and Noob Saibot. However, these costumes are modeled after their MKII costumes rather than the more simplistic MK1 outfits as indicated by their masks.