Who started the custom of Elijah’s cup and that he visits every Seder on Passover night?

There is a halachic question as to whether it is good to drink a fifth cup on the seder night (the Shulchan Aruch mentions only four; Rambam writes that a fifth is not required but is customary). Elijah is the one who will clarify halachic issues like this. Also, the cup reminds us of the full redemption which we await. Elijah is the harbinger of that redemption.