During todays Persona X-Steam Special Episode, it was announced that the female protagonist from Persona 3 Portable will be added to Persona 5: The Phantom X in a future update.
She will use the name Kotone Shiomi, which has previously been used as her name in the Persona 3: The Weird Masquerade stage shows, and a Puzzles & Dragons collaboration.
The character is still early in development and so has only been revealed as a silhouette. More details on the event will be revealed at a later date.