anyone who spoils a good time, breaks up a party, or affects your life or your situation in a negative way. Often times they are synonymous with downers, or party "poopers" for you elderly people.
IF you are partying in your house and having a great time with your friends, and another friend walks in and proceeds to be a downer and does nothing but complain and spoil the mood they are being a RUINER. Feel free to call them on it.
"You are being such a ruiner right now"