Red Rain is a story-related weather phenomenon introduced in God Eater 2, which literally as it sound, crimson clouds raining down crimson rain . It will not appear while or during missions, as it merely for story plot.
A weather phenomenon which began within the last six months (of the year 2074). First recorded in the Far East, it is named for the red Color of the raindrops.
When a human comes in physical contact with this rain, there is a high likelihood that they will contract the black plague.
A brief time after the rain has fallen, the red color vanishes and it changes into regular rainwater. The cause is still unknown