First emerging as a general in Shinnok's war against the heavens and an apparent member of the Brotherhood of Shadow. Reiko has served both the fallen Elder God and Shao Kahn.
Believed dead after Shinnok's first defeat by Raiden, he emerged during the events of Mortal Kombat 4 once as an ally of his former master. However in the wake of his masters second defeat at the hands of Earthrealm's defenders, he disappeared once more.
Reiko later emerged as a general in Shao Kahn's army having gained promotion to the position due to his ruthlessness. His ambitions seem to go much further however as his desire to take the emperor of Outworld's throne is all too clear.
In canon, he served as both Shinnok and Shao Kahn's general in their armies at different points in time.
Flanderization: Not that he had much characterization to begin with, but Midway took his connection to Shao Kahn and ran like hell with it. In Deception, he claims he enjoys sneaking into Kahn's throne room and wearing his helmet. Come Armageddon, he's gone from maskless ninja to a helmetless copy of Kahn (except with proper pants), complete with his weapon being changed from a war club to the Crude Hammer, further emulating Kahn. One of his attacks even generates an aura version of Kahn's helmet. And what does he do in his Armageddon ending? Defeats Shao Kahn and claim his helmet, which fuses to his head.
His MK4 ending has him step through a portal to a throne room and sit, and then Shao Kahn's helmet magically lowers itself onto his head. At this point it was strongly implied that Shao Kahn was dead, and Reiko's bio says that he too was supposed to be dead. So the idea was that Kahn had cheated death by possessing Reiko's body, or something along those lines. The fans generally either did not like this, or did not get it (until Deception, many people believed that this meant he was Shao Kahn), and the idea was scrapped, and in Deadly Alliance it was decided to make it that Kahn had never been killed in the first place. The helmet thing is thus a Running Gag. Originally he never even served Kahn; he had only worked for Shinnok.
Flechette Storm: Throws a barrage of shuriken at his opponent for his Fatality in 4. Noob Saibot's appearance in Deception and his shuriken's Relic description in Armageddon affirms that such weapons are signs of membership in the Brotherhood of Shadow.
Multicolored Hair: In 4, his hair was mostly black, with some white on the sides. In Armageddon, the sides have been shaved off.
Stalker with a Crush: His relationship with Kahn comes off like this, especially the creepy sneaking into his throne room and wearing his helmet. In Armageddon his outfit's even similar...
The Starscream: Possibly to Shao Kahn. He certainly wants that helmet.
"I had been called an enforcer of the Deadly Alliance. I preferred the term 'ambassador.'"
The daughter of an Edenian ambassador to new realms, Tanya allowed fleeing refugees to enter Edenia in order to find safety. However these unfortunate victims turned out to be Shinnok, Quan Chi and their deadly Netherealm forces. Tanya had betrayed her own realm to the fallen god.
Despite attempting to lure Liu Kang into a trap, she was eventually undone with Shinnok's defeat at the hands of Earth's warriors. Her treachery known to both Edenia and Earth she fled to Outworld. Eventually she found herself an an enforcer for the Deadly Alliance and imposing the will of her masters on Outworld.
Their defeat left her contemplating a return home but after a confrontation with Baraka was drafted into the ranks of the Dragon King Onaga's army. Later bringing her new master to her native realm in search of information on the Kamidogu.
Beauty Is Bad/Face of an Angel, Mind of a Demon: Deception's Konquest Mode even specifically states, "Tanya is beautiful, but she is not to be trusted." Considering that this angel-faced dame went on to betray her Edenian and Earthrealm allies, this is understandable.
Heel Realization: Possibly, in a sense. Her bio for Deception notes that Tanya, now lonely after the fall of the Deadly Alliance, felt an urge to return to her homeland of Edenia (y'know, the same realm she betrayed back in 4?). However, this is where the Tarkatan horde intercepts Tanya and forces her to lend her talents to Onaga's cause.
Invisibility: Gains a move like this starting in Deception. Appropriately, the technique is named "Find Me."
Kiss of Death: Like Kitana and Sonya, she had a similar move as a Fatality in 4 (called Kiss of Deceit; perhaps a nod to her true nature). This fatal peck causes the receiver's body to agonizingly contort before exploding.
Spin Attack: In Deception, a special known as the Human Cannon Drill is added to her repertoire. It involves Tanya corkscrewing under opponent's legs like... well, a drill.
Summon Magic: Her ending in Armageddon has Tanya transforming into a being known as a Dragon Caller. With the ability to conjure ethereal dragons and give them a corporeal form, Tanya then goes on to conquer all of the realms with her new army.
"I will not fail."
An-ex member of the secret White Lotus Society. Kai gained his skills learning from the great masters throughout Asia. After a chance meeting with Liu Kang looking to train future Shaolin warriors, Kai joined him in China for further instruction.
However his training was halted by the invasion of Shinnok's forces. Kai then joined the other Earthrealm warriors to battle alongside Raiden against the fallen Elder God. After Shinnok's defeat, he decided to walk the Earth in search of self-enlightenment. As a sign of thanks and approval Raiden entrusted him with his Lightning Staff to help guide him on his quest.
Ambiguously Brown: An odd sort - he is Nepalese, but his skin tone and facial features more resemble that of a Black African.
Death from Above: His fireball move in Mortal Kombat 4. He can also invert this and shoot the fireballs downward, causing them to shoot up out of the ground under his opponent.
Kukris Are Kool: His weapon in 4, although he switches to Raiden's staff after completing arcade mode.
After learning of Kano's apparent demise, Jarek fled to the realm of Edenia. Pursued by special forces members Jax Briggs and Sonya Blade who believe him to be the last member of the criminal Black Dragon clan. However their plan to bring him in is halted by Shinnok's invasion of the realm. This leads Jarek to reluctantly join forces with them and the rest Earthrealm's warriors to deal with the greater threat.
After Shinnok's defeat Jax throws Jarek to his death over a cliffside. Jarek managed to break his fall by digging his blade into the rock face. However his injuries were still substantial. The time taken to hone his skills and recover, whilst presumed dead, did little to lower his bitterness over having to overcome his wounds. Eventually his desire to kill Jax extended to everyone.
Enemy Mine: Teams up with Sonya and Jax (and the rest of Earthrealm's warriors by proxy) to defeat Shinnok in 4. It takes all of five seconds after the Elder God's defeat before Jarek and Sonya are at each other's throats again.
Eye Beams: In MK4, because of his previous status as that game's Kano. In an effort to include more new characters in-game, they reskinned Kano as Jarek (hence the reason Jarek has mostly Kano's moves).