The lawyer first appears in Little Dipper. He comes to the Mystery Shack claiming to have a million dollar cheque for Stan until it is revealed to all be a trick by Gideon in an attempt to get the Mystery Shack.
Alex's Twitter Prank
Just before the premier of Little Dipper, Alex Hirsch posted on twitter a picture of the "Unnamed Lawyer" saying it was "Mitt Romney's appearance in an Upcoming Gravity Falls Episode" and that Grunkle Stan and he would bond over a "love of not paying taxes". He later posted that the comment was a prank.