Ruben (ルチル Luchil) is a little boy who lived a long time before the main events of Pandora's Tower. He appears in Elena's dreams which occur after she eats the Master flesh. He lived happily with his father and mother, and even though there was a war going on they did everything they could to keep him happy. They even gave him their food, for food was very scarce during the war.
In her sixth dream Ruben ran off to watch some knights who are marching through their village, because he had never seen horses before. His parents are worried because they can't find him anywhere, and they ask everyone they see. Eventually they get to where the parade was, only to discover some horrible news. Ruben had dropped his toy horse on the road before the parade began. When the knights were coming along on their horses he realised this and ran out in front of them to get it. At first, no one wanted to stop the parade. He was trampled to death by the horses. His parents are devastated when they find out.
These events are what caused his mother and father to volunteer to be Vessels, which lead to them being chosen for Experiment Zero . According to the S Ending, eventually his parents did have another child, although his mother was lost in Experiment Zero, meaning she didn't get to raise him. Ruben's father declined from the experiment, took him and raised the child, and Ruben's younger brother became one of Aeron's ancestors.