Kadrian Sey was a female Zabrak Jedi Knight. Prior to her fall to the dark side, she served within the Jedi Order. Subsequently, she aligned herself with the Confederacy of Independent Systems during the Clone Wars, becoming one of Count Dooku's Dark Acolytes.
Identified as Force-sensitive in her youth, Kadrian Sey underwent training at the Coruscant Jedi Temple's academy, with the goal of mastering the Force. After constructing her own lightsaber and embracing the traditional facial markings of her Zabrak heritage, Sey was chosen by a Jedi Master for advanced instruction. Upon completing her Padawan apprenticeship, Sey achieved the rank of Jedi Knight and was assigned extended missions in the Outer Rim Territories by the Jedi High Council. Consequently, she was absent during much of the Separatist Crisis that engulfed the Republic in the years leading up to the first battle of Geonosis, marking the start of the Clone Wars.
When the High Council summoned all Jedi, ordering them to assume military roles in the Grand Army of the Republic, Sey disobeyed the summons, never returning to Coruscant. Forsaking her Jedi path, the Sith Lord Darth Tyranus swayed Sey to the dark side, inviting her to serve as a Dark Acolyte. As a member of Tyranus' personal guard, Sey forged a red-bladed lightsaber, which she skillfully wielded alongside fellow acolyte Tol Skorr.
Tyranus, operating under his alias Count Dooku, the leader of the Confederacy, led Sey, Skorr, Quinlan Vos, and Sora Bulq on a diplomatic endeavor to liberate Tibrin. Upon entering the royal palace, the group confronted the planet's ruler, Suribran Tu, who promptly offered to betray the Republic and support Dooku's cause. The aged Sith Lord sought counsel from his companions regarding their next action. Sey advocated for accepting the offer, arguing that it would ensure governmental stability and facilitate a smoother transition to the Confederacy. Vos, a fellow acolyte, countered by asserting that leaving Tu in power, given his reputation as a despised tyrant, would only reinforce the perception that the Confederacy was no different from the Republic. Dooku sided with Vos' argument and beheaded Tu without further deliberation.
Sey also traveled with Dooku to Kiffu, where the Count was scheduled to meet with Sheyf Tinté Vos, accompanied by Tol Skorr and Quinlan Vos. When negotiations deteriorated, Dooku instructed the acolytes present to assassinate the Sheyf and eliminate her guards. At that moment, Vos revealed himself as a Republic agent, engaging Sey in lightsaber combat and bisecting her at the waist to safeguard the rightful leader of the planet.