In general, a vertical stabilizer connected with the fuselage will form junctions. The junctions trigger a mix of airstreams, and causes interference drag. A fairing is used the ease the sharp junction angle and reduce this type of drag. Like between the junctions of the aircraft's main wings and the fuselage.
However, on an B787 aircraft, it is not easy to discover the fairing installed for the vertical stabilizer. Does the 787 really have this fairing? If not, what stops the manufacturer from adding this component, and reducing drag? Does the 787 have this fairing? Image source:https://www.alamy.com/stock-photo/boeing-787.html?sortBy=relevant