Rayden

Name: Rayden
Origin: Unknown (Heaven?)
Race: God
Playable Debut: Mortal Kombat
Unplayable Debut: Ultimate Mortal Kombat 3 (arcade) (getting really technical here)
Story: Rayden's story begins long before most. Rayden was appointed to guard the new realm of Earth long ago by the Elder Gods. Shinnok, an Elder God, tried to take the realm. The battled that ensued between Guardian and Conqueror immersed the new realm into centuries of darkness.
Rayden eventually defeated and captured Shinnok and brought him before the Elder Gods. They decided that he be banished to the most feared place in all universes: Hell; Hades; Pluto . . . or the Netherealm.
Shinnok had used an amulet to gain access to earth, but the power it took to create it could only be generated once, so only one amulet would ever exist. Rayden took the amulet and locked it deep within a temple in the Himalayas, guarding it with four elemental gods: Water, Earth, Fire, and Fujin of Wind. The entire story was soon lost over generations except for one document which came into the hands of the Shaolin monks: the Map of Elements.
Centuries later, in 1983, Rayden learned that a Lin Kuei ninja, Sub-Zero the Fifth, had managed to steal the Map and get inside the temple, defeat the four gods, and steal the amulet. And if that wasn't atrocious enough, he had given to amulet to a free-roaming sorcerer named Quan Chi; the sorcerer of Shinnok!
In a fury, Rayden banished Sub-Zero to the Netherealm and told him that he could only be granted leave if he retrieved the amulet from Shinnok. Sub-Zero succeeded in his mission.

Ten years later, Rayden was invited to Mortal Kombat by Shang Tsung. Rayden knew that Shang Tsung was treacherous and worked for Kahn, but accepted nonetheless, knowing that Tsung only wanted to get him to become mortal, as only mortals could participate in the tournament, and get him out of the way when he won the tournament for the tenth time and Kahn was able to take over the Earth.
Liu Kang won the tournament and Kahn's streak was broken. With it, Rayden returned to the heavens. The following year, he saw the earth warriors venture into the Outworld and looked into it.
Upon doing a little interrogation and research, Rayden found out what was being planned; Shao Kahn's invasion was imminent, and these various plots to lure the deadliest of the Chosen Warriors was merely to get them out of the way when he seized the realm.
Warning the other members of the former Shaolin tournament, Rayden took steps to ensure their safe return to the earth realm. Then the invasion began.

The Queen of Edenia, Sindel, now transformed into a scheming wife of Kahn by Shinnok, was reborn on the Earth realm after committing suicide centuries ago. With her on Earth, Kahn was able to step through the dimensional gates to reclaim his queen. When he did that, Rayden and the other gods could do nothing but watch in horror.
(Note: If you read it closely, you'll find that the bios and endings for Rayden and Classic Sub-Zero can still fit with the following theory)

Upon seizing his queen, Kahn stripped the souls from every living creature, and Rayden could no longer stand it. The Elder Gods could do nothing, but Rayden refused to take no for an answer. He transformed into the most skilled warrior he could recall: Sub-Zero the Fifth. In doing so, his own immortality is at risk.
With his assistance and guidance, Shao Kahn is no match for the tenacity of Earth and falls. With that, Rayden returns to the heavens as Earth returns to normal.

Two years later, Shinnok, with the help of Quan Chi and the now dead Shang Tsung and Shao Kahn, invades Edenia, where Queen Sindel, who regained her memory and took her rightful place, has taken the throne with the advice of her daughter, Kitana. And with an army led by Quan Chi attacking the heavens, chaos rules.