Rain is from the ancient realm of Edenia. As a little boy, he bore witness to the invasion of his home world at the hands of Shao Kahn. During the chaos that ensued, Rain was smuggled away while his father stayed behind to perform his duties as a general in the Edenian army. Edenia's armies were crushed in the Outworld invasion, and Rain's father was subsequently killed by Kahn. He is half-human and halod.
Thousands of years later, Rain mysteriously returned during the invasion of Earthrealm and was attacked by Kahn's extermination squads. Given the choice to fight for Kahn or be killed, he knowingly turned his back on his homeworld, and he chose to side with Shao Kahn. Either out of fear, cowardice, or ulterior motives, he aided Kahn in his plans for the conquest of Earthrealm. Trained in the art of kombat, Rain fought whole-heartedly to destroy Earth's warriors.
As Rain has not appeared as a playable character in years, and Deadly Alliance offered no insights to his current whereabouts, details about Rain's fate remain unknown. His Mortal Kombat Trilogy ending stated that Kitana confronted Rain and was able to turn him against the Emperor by informing Rain of his father, an Edenian general's stand against Shao Kahn leading to his subsequent death. After learning this, Rain was furious, and embarked on a one-way suicide mission to stop Kahn.
Rain appears as a non-playable character in Mortal Kombat: Deception, in Konquest mode. He can be found in both Edenia and Outworld, and is involved in two separate sub-quests to be undertaken by Shujinko. In the second sub-quest, Rain is shown to have been slain by Jade. It is uncertain how many different events taking place in Konquest mode are canon.
During the events of Mortal Kombat: Armageddon, Rain learned of his true Edenian heritage. Quan Chi informed Rain that he is actually a direct descendant to Argus, the protector god of Edenia. Rain then started to refer to himself as a prince of the realm, as seen in Armageddon's Konquest mode, but he still chooses to fight on the side of evil, although he no longer serves Shao Kahn. He wishes to kill both Taven and Daegon in order to be able to win the ultimate prize and power promised from defeating Blaze. Rain confronts Taven in Arctika, but he is beaten in battle, eventually escaping in a watery portal.
In Rain's Armageddon ending, it is revealed that he is actually the half-brother of Taven and Daegon, being the son of Argus whose identity his father had tried to keep hidden. Imbued with the power of Blaze after defeating him, Rain becomes a full god, and Argus proclaims him as the new Protector of Edenia. However, Rain instead uses this power to enslave the realm. Argus, having ascended to the status of Elder God after Rain's own ascension, is unable to stop him.