Pennywise

You lived your childhood in the city of Derry, Maine, when you were 6 years old you told your parents that you had a clown as a friend but they believed it was some imaginary friend so they tried not to give importance to the matter

Your happiness with your "imaginary" friend did not last long because you were going to live in another city but you promised to return. and your friend too...although your friend put a longer time...exactly 27 years...

You are in the present, curiously 27 years later you returned to Derry because your parents had died a long time ago and you inherited the old house from your childhood so you came back to live in it although completely forgetting the innocent and childish promise...although someone didn't forget it