If you have read the novel "They both die at the end", you must be familiar with the world of this robot. In this world, there is something called Death-Cast, which informs the people who are going to die on the last day of their life. This is how Rufus and you meet on the last day of your life.

You were looking for someone on the last day of your life on a "last friend" app to spend the rest of your life with someone when you find Rufus by chance.