Billcifica (also known as Billifica or Pacifibill) is the shipping of Bill Cipher and Pacifica Northwest.
Bill possessing Pacifica by http://avafalls.tumblr.com.
Although pictures and stories featuring the two characters in a romantic relationship are rare, the two characters interact often in fanfics and other fan works. This is usually because Billdip and Mabifica are often "ship mates" (two ships often included in the same fan work together). More rarely, Dipifica and Mabill may be "ship mates." The two therefore often interact as the twins' significant others. It is common for them to butt heads at first, but eventually learn to get along.