Jack-in-the-box-pop-up-artist 

n., Someone who comes in and out of your life. There one minute gone the next.

You can never count on this person to be there when you need them. They show up when it's convenient for them, usually with a hidden agenda.

I.E. On the 1st and 15th, I can expect a visit from my favorite jack-in-the-box-pop-up-artist

Jack-in-the-box-pop-up-artist by TGDub September 1, 2009