Rain is an Edenian who joined Shao Kahn during MK3 because he was afraid of being killed. However, Kitana told Rain Kahn killed his father when he conquered Edenia so Rain joined Earth's forces.
However, he disappears during a battle and everyone assumes he's still working for Kahn. To prove his loyalty to Kitana he goes on a suicide mission to destroy Shao Kahn.
What happened after that we don't know.
He could return and try to help free Kitana like Jade and Sindel, assuming he's alive and still good.

thx.....OMG!..i just remembered!...rain is in kahns possy with ermac, motaro etc in konquest..i have seen screens...maybe we will find out in this one