An embodiment of Sephiroth's will manifested from the Lifestream, his goal is to find Jenova, whom he considers his mother, and bring about a second Jenova Reunion. Like the other embodiments of Sephiroth's will, Kadaj resembles Sephiroth to a great degree and is described by Vincent Valentine as a "larval" form of Sephiroth. Kadaj wears black leather clothing with gauntlets and a coat, which he keeps zipped up. The image of a single wing is sewn on the back of his coat as an allusion to Sephiroth. He has shoulder-length silver hair that partially covers his face and green cat-like eyes. Kadaj's weapon is the Souba, a katana with two parallel blades that he carries in a sheath on his lower back.
Kadaj embodies Sephiroth's cruelty and insanity. He is devoted to Jenova and dedicated to finding her, but is unaware of Sephiroth's influence over Jenova. As a result, Kadaj is insecure over Jenova "choosing" Sephiroth over him, and though he does not fully understand who Sephiroth is, he tells Rufus Shinra he can sense his presence. Kadaj is arrogant and mocks Rufus and the Turks, and thinks of Cloud Strife as his "brother" through Jenova, but considers him a traitor to their cause. When driven to anger Kadaj attacks with fury, such as when he attacks Cloud when Cloud reminds Kadaj he is only Sephiroth's pawn. Nomura describes him as "talking quickly, then speaking slowly or breaking into laughter for no apparent reason"
Kadaj is a highly skilled swordsman and battles Cloud competently one-on-one. When Cloud is weakened due to Geostigma, Kadaj can defeat him, and is strong enough to defeat both Reno and Rude alone. Kadaj can summon hordes of Shadow Creepers from the Lifestream, and uses Materia to summon Bahamut SIN. In Advent Children Complete, Kadaj infiltrates Healen Lodge by transforming into a black mist. Upon walking into the water at the Forgotten Capital it turns dark and the Jenova cells within the children become active when they drink it. This may be a form of Limit Break, as Kadaj glows in a manner similar to other characters in the film when they use their Limit Breaks.
Kadaj appears before certain individuals under the guise of deceased persons from their past to manipulate them to help him search for Jenova, reminiscent of Jenova's transformation power. He can vanish and reappear in another location, and to become immune to attacks from firearms, likely extensions of his ability to disperse into mist. He can take away people's physical pain as well as re-grant it, as evidenced by his "mending" Evan's shoulder wound and his encounter with Evan in the mountains. Unlike most antagonists with great physical and magical power Kadaj does not have Jenova cells, and not acquire Materia until halfway through the events of Advent Children. His unusual magic abilities are derived from the negative Lifestream that makes up his body, while his great physical strength and combat proficiency come from being a personification of Sephiroth's memories.
Kadaj was born in a pond of blackened water near Nibelheim. He saves Kyrie from sinking in the pool, and ends up conversing with her at Dr. Drake's office, with the latter seeing him as a friend who died in a fire, due to Kadaj's ability to mimic the appearance of one's dead relatives, a trait he shares with Jenova. Kadaj later disguises himself as Dr. Drake and Fabio, and takes away Evan's pain from the surgery when his arm was shot. When confronting him at a cabin on the mountain, he gives him back his ability to feel pain. Kadaj later attempts to manipulate Kyrie into supplying him information on Jenova's remains, but Kyrie shoots at him. Kadaj attacks a Turk helicopter and tricks it into shooting itself down with a missile. He goads Kyrie into going into one of the crevasses and pursues them in the caverns, only to end up diving into the Lifestream and dissolving after Evan kicks the still-living remain of Jenova into it.