Gematsu have posted an article with details from the special E3 2016 Niconico live stream for Persona 5, where Atlus have revealed the 9th party member, named Goro Akechi. Similar to Naoto in Persona 4, Goro is a high school student but also a detective. The character will be voiced by Sōichirō Hoshi.
Also revealed were three “Cooperation” characters, which we caught a glimpse of in the trailers earlier on. These characters aren’t part of your team of Phantom Thieves, but are side characters that you’ll get to learn over the course of the game. In Persona 4, you could establish a “Social Link” with some of the side characters you meet, which allowed you to unlock Personas and use them in battle. In Persona 5, it appears the Cooperation characters are the equivalent of the social links in other Persona games. Establishing a bond with these characters will allow you to use their special skills to your advantage in battle. You can view new videos of three Cooperation characters below.
Persona 5 is scheduled to be released on the PlayStation 4 and PlayStation 3 on September 15 in Japan. The release in the Americas is scheduled for February 14. Nothing is know concerning a European release at the moment.