The relationship between Will and the Upside Down has existed since the first Stranger Things episode. Will rides his bike home after a long game of Dungeons and Dragons only to be stopped by a Demogorgon on the prowl en route. He flees, lands in the Upside Down, and is able to remain hidden for the majority of the season before being ultimately saved. The Upside Down is still tied to Will in Stranger Things season 2, with his experience there and the particles of the dimension that entered him working as a kind of anchor that continues luring him back.
In the first four seasons of Stranger Things, Will is frequently the secondary character. He is barely present in the show, despite being a significant focus in season 1 owing to his situation of being trapped in the Upside Down. Will has greater screen time in season 2, although his true purpose is to serve as the Mind Flayer's vehicle. In Season 3, he is utilised far less frequently because the season's increasing action takes precedence over a plot about him feeling abandoned by his pals. In contrast, Will's character is utilised more and given some impactful moments in Stranger Things season 4, yet he is still definitely there in the background.