Rain is from the ancient realm of Edenia. He hails as the child of Argus, Edenia's protector. It is currently unknown who his mother is, however Argus's shame of infidelity, caused him to exile Rain from his royal family. As a result, Rain was given to an unnamed, but key general, in the Edenian military. Rain is a demi-god, the son of a true god and an edenian.
His actions may have began before the events of the 1st Mortal Kombat tournament, as depicted in Mortal Kombat Deception's Konquest Mode. Rain appears in Mortal Kombat: Deception as a non-playable character and is involved in two separate side quests. One of which involves Rain asking Shujinko to retrieve the enchanted dagger of the royal Shokan family. Rain states that Shao Kahn has enlisted him to retrieve the dagger for his own purposes, but Rain actually plans on keeping it for himself. Although this quest ends in Jade knocking out Rain for his troubles, it is canonly believed that he did keep the dagger, as it appears as part of his wardrobe in Mortal Kombat: Armageddon.
Several years later, Rain mysteriously returned during the invasion of Earthrealm. 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 but his ending has Princess Kitana informing him about the true death of his adoptive father; that he was killed by Kahn's own hands. Overcome with a sense of rage, Rain embarks on a suicide mission to avenge his "father"'s death. It is unknown if this ending is canon or not, as the ending does not state if Rain succeeded or not. Merely that he left the Earthrealm Warriors to embark on the quest. Or it could be that Kahn was defeated by Earthrealm warriors before Rain confronted Kahn.
As Rain has not appeared as a playable character during the events of Mortal Kombat 4, Mortal Kombat: Deadly Alliance, or a main portion of Mortal Kombat: Deception, his current whereabouts were unknown. However, 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.
In Mortal Kombat (2011), the official reboot of the series, which covers, and alters, the past events of Mortal Kombat 1-3, Rain was introduced as DLC. His original backstory as an Edenian orphan is retained, although in a modified form.
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 himself.
After that, he was rescued and raised by the Edenian Resistance following his adoptive father's death. He trained hard under their tutelage and turned into an elite but arrogant fighter. He served the Resistance well for many years and he was denied his own command. Infuriated, he betrayed his own brethren and joined his mortal enemy, Shao Kahn, who promised Rain leadership of his own army.