D9, known as Deenine, was a protocol droid spy and an associate of the self-made droid L3-37. At L3-37's request, he tracked down the Phylanx Redux Transmitter, a device created by the gangster Fyzen Gor that could control droids, forcing them to kill. D9 was captured by Gor, and was left mutilated in the gangster's workshop. L3-37, accompanied by the smuggler Lando Calrissian, followed a signal sent by D9, and learned of the Phylanx Redux Transmitter's nature. After meeting with L3-37, D9 malfunctioned due to damage sustained at the hands of Gor.
By around 11 BBY, D9 was an associate of L3-37, a self-made droid who traveled as the partner of the smuggler Lando Calrissian. After L3-37 heard rumors of the Phylanx Redux Transmitter, a device that could take control of droids, D9 sought to find information on the device. While D9 managed to learn that the device had the ability to change the programming of droids and force them to kill, the Pau'an gangster Fyzen Gor, the creator of the Phylanx, eventually captured and mutilated the protocol droid, whose dilapidated body ended up in a pile of droids aboard Gor's workshop, a floating metal chamber in the Mesulan Remnants asteroid belt.
D9 managed to send a transmission to L3-37, who, along with Calrissian, visited the Mesulan Remnants Belt in their starship, the Millennium Falcon. L3-37 found D9 among the droid wreckage, activating him. Before revealing any information, D9 informed L3-37 of a second spy in the chamber, a Galactic Empire astromech droid. Calrissian destroyed the astromech droid, and D9 warned L3-37 of the Phylanx that Gor was constructing. As he spoke, D9's voice began to malfunction, turning into nonsensical beeps.
D9 was a protocol droid with orange photoreceptors. D9 had become severely damaged by the time that L3-37 tracked him down, but was more concerned with warning her of the Phylanx Redux Transmitter.
D9 appeared in Last Shot, a 2018 novel written by Daniel José Older.