But what can we do? We're just remnants... merely remnants of Mother's legacy. Until we find her and receive her cells, we can't be whole again!
The Remnants of Sephiroth (セフィロスの思念体, Sefirosu no Shinentai?) (also known as the Silver Haired Men) are a trio of young men that serve as the primary antagonists of Final Fantasy VII: Advent Children. Formed by Sephiroth as physical manifestations of his will from within the Lifestream, the remnants are each a fragment of his will and serve to enable his physical resurrection into the world. The original Japanese word used is Shinentai (思念体) which literally means "physical thought" as thought or conscious given physical form.
The remnants of Sephiroth are Kadaj, Loz, and Yazoo. Kadaj is the representation of Sephiroth's hatred and rage and serves as the leader. Loz is the manifestation of Sephiroth's physical strength and speed. Yazoo represents Sephiroth's charisma and aloof demeanor, although without Sephiroth's other traits this makes him appear and act effeminate.
Of the three, Kadaj resembles Sephiroth the most as he uses a sword and acts the most like him. All three wear black, have silver hair and are left-handed, making them similar to Sephiroth to a certain degree. The remnants are implied to help strengthen Sephiroth's spirit within the Lifestream by finding others who know of him and reminding him who he used to be in life.
The remnants indicate being different from Sephiroth, as Kadaj claims to Rufus he does not even know Sephiroth, but can sense his presence and is distraught over "mother" choosing Sephiroth as her son over him. Loz and Yazoo do not indicate one way or the other if they are aware of Sephiroth's existence. While the remnants wish to find Jenova, they do not appear aware of why they want to; Loz and Yazoo tell the Turks "she'll fix everything" and other equally-vague statements. Kadaj tells Rufus that when they find "mother" they will conquer the Planet.
Kadaj appears not to know what to do with Jenova's remains when he acquires them. Upon Cloud explaining "a remnant wouldn't really know", Kadaj does not object and bitterly asks "So what if I'm a puppet?", indicating he is aware of his ignorance.
The remnants refer to Jenova as "mother", indicating that, like Sephiroth before the Nibelheim incident, they believe Jenova is their actual mother. Although they know her true name, as shown by their reactions to Reno's comment about Jenova during their battle with him and Rude, they are otherwise unaware of her state. After Rufus shoots the box containing her remains—which is little more than a few remaining cells—Kadaj is horrified upon seeing what is left of Jenova.
What the remnants had originally planned to do with Jenova, is unknown, if they had any plans to begin with. Through Geostigma Jenova had infected many people, mostly children, with her cells. Kadaj indicates to Rufus and the children that this means they are, in essence, Sephiroth Clones, and thus needed for the "reunion", as the Remnants themselves have no reunion instinct. Kadaj, in desperation during his battle with Cloud, absorbs the remains of Jenova and transforms into Sephiroth. This may have been the remnants' purpose all along, even though they were not aware of it.
In the On the Way to a Smile novella "Case of the Lifestream", it is revealed that Sephiroth tried to create an avatar of himself, but as his memories are fragmented he is unable to. Sephiroth searches the Lifestream for other memories, and uses them to craft the three remnants to search for Jenova so he may be fully reborn. Final Fantasy VII: Advent Children Complete establishes that the Remnants are less than two weeks old chronologically during the time they attack Edge.
In Final Fantasy VII Lateral Biography Turks -The Kids Are Alright-, a novel that delves into the events prior to Final Fantasy VII: Advent Children, Kadaj operates alone, killing Dr. Drake and stalking Evan Townshend and his partner Kyrie Canaan while they search for Evan's missing mother, Annette. They learn that Annette had been sent north of Icicle Inn to recover the Sephiroth Clone test subjects who had gone missing from Nibelheim.
After battling the Turks on the Great Glacier, Kadaj pursues Evan and Kyrie into a crevasse that opens up beneath the snow, leading to a Mako cavern that contains the dead bodies of Annette and the test subjects. Among the possessions Evan finds on her body is a small piece of Jenova's still-living flesh wrapped in a handkerchief; Evan kicks it into the Lifestream and Kadaj dives in after it, dissolving.
The remnants travel to the outskirts of Midgar where they attack Cloud. Afterward they interrogate Rufus about the whereabouts of Jenova. In the city of Edge, the remnants kidnap several children with Geostigma—a disease caused by an infection of Jenova cells—and take them to the Forgotten Capital, where Kadaj activates their latent "reunion instinct".
With the children's ability to sense Jenova's presence, they lead the remnants back to Edge, and the trio attacks the city to force Rufus to reveal where he has hidden Jenova. Rufus reveals Jenova's remains under his cloak during the battle, which Kadaj salvages. In the ensuing chain of events Loz and Yazoo are apparently killed, and Cloud battles Kadaj alone. Desperate, Kadaj absorbs Jenova's cells, allowing Sephiroth to take control of his body and reshape it into his own image.
After Sephiroth is defeated, wounded Kadaj hears Aerith Gainsborough's voice, whom he believes to be Jenova. Aerith comforts him and calls down healing rain, and Kadaj, while reaching for a hand only he can see, allows himself to dissolve into the Lifestream. Loz and Yazoo unwillingly follow soon after, attempting a final attack on Cloud. The three remnants die, although it is unknown what this means for Sephiroth himself.