Rain is the alias used by Rein Kashiwagi, a thirteen-year-old middle school student who participates in the Darwin's Game as an information broker. She makes her living inside the game by buying and selling information, and she is known among other players for the accuracy of her analysis. Because she can examine collected intelligence and draw precise conclusions from it, players call her the Analyst. She becomes a core member of the Sunset Ravens, the group centered on the protagonist Kaname Sudou, and she stands as one of the central figures supporting the main cast throughout the story.

In terms of background, Rain is a first-year middle school girl who, like other participants, was drawn into the game's world and found her place in it through knowledge rather than combat. She is calm by nature and tends to handle situations logically, always thinking matters through before she acts. She has little interest in fighting and avoids direct conflict whenever possible, once describing herself as someone who only runs away rather than fights. For her, surviving through careful decisions is more important than winning battles, and this attitude shapes both her behavior and her approach to the game. She has also reflected that it sometimes feels as though two versions of herself exist at once, one that responds emotionally and another that calmly watches from a distance, a sensation she compares to depersonalization.

Her main motivation is to survive the Darwin's Game by relying on information and strategy instead of force, and she supports those around her when doing so strengthens the chances of survival for the group she belongs to. This motivation leads her to gather extensive knowledge about the game's rules, its events, and the abilities of the players, and to use that knowledge to guide her allies.

Rain's notable ability comes from her sigil, which is called Worldwide Function. The sigil is described as analogous to Laplace's demon, a hypothetical being that knows the exact location and momentum of every atom in the universe. In practice, this grants her a special power to predict movements, allowing her to anticipate what others will do and to turn gathered information into accurate forecasts. This ability, combined with her sharp intellect, makes her an invaluable asset as an analyst and strategist rather than a frontline fighter.

Within the story, Rain serves as the information backbone of the Sunset Ravens, working closely with Kaname Sudou and the other members by supplying the intelligence needed for their operations. Her key relationships are built around this role, as players on all sides recognize her value as a broker whose information can change the course of events. Over time, she develops from a cautious broker who prefers to stay on the edges of conflict into a more involved and responsible figure within the game's larger structure. Later in the story, after the death of Shidou Izaya, she takes on the role of Guardian of the World Branch of World Line N, a position that places significant responsibility for the game's world lines in her hands.