What if Rin was never alone in the first place?
What if, when Shigeru (Rin’s dad) was building the capsule for his daughter, another family found out about his technology and begged him to save their 8-year-old son, to which (after much pleading) he agreed?
What if Shigeru’s plan was to have them meet someday, after Rin received his letter to her?
What if that day comes, and Rin meets him?
Him, being the one and only Porter Robinson.
Which explains why both Worlds and Shelter are about two people: (in Worlds’s case) a boy and a lonely girl.