The actor Ben Schwartz appeared in aLate Late Showspot to solve the mystery.
Ill never tell, he answered, before nodding obviously.
Then Jean-Ralphio Saperstein(-Harrington?)

Credit: Curtis Baker/Netflix; NBC
entered the world in a flashy way and immediately enacted aget-rich scheme.
He came out, and got run over by an ambulance to make money.
He was super chill, super cool in the Upside Down.
He ran that s, Schwartz said.
He was super friends with the monster.
When he met the monster, it was like a little kid.
By the time it was a full-blown monster, they had a dance together.
The monster didnt even open yet; it was a sprouted flower.
FormerParks and Recshowrunner Mike Schur confirmed the theory in no uncertain terms:
See the video below.