In Final Fantasy Advent Children who is Kadaj?

Kadaj is a Remnant of Sephiroth just like his 'older' brothers Yazoo and Loz. They are on a constant search for Jenova, who they refer to as Mother. He does this because he was created by Sephiroth's will that was residing in the LifeStream. But, don't take this to mean that they are the same. Kadaj and his brothers are indeed separate entities from Sephiroth, but are somehow still the same. Some of this is revealed through Advent Children Complete, as well as in some written information Square Enix has released entitled 'On the Way to a Smile - Lifestream Black and Lifestream White'.
"He had decided he would use his Mothers power. With a fragment of Mothers body, I too can get a body again, the man thought. And so firstly, he tried to manifest on the surface as just a spirit, but his attempt failed. He had already returned the memories of his own appearance to the planet, and so he was not able to produce an image of himself. So the man found memories of a suitable appearance from the Lifestream, and with that form produced an image. It was the form of a boy. Soon the man remembered that being on the surface was incomparably more limited than the freedom of being a spirit. He created two more agents to do his work. These three were separate entities, and at the same time he himself. These three, created by the strength of the mans will and detached from the system of the planet, were both at once a part of reality and monsters out of fantasy."

Kadaj info:
Name: Kadaj
Age: 16
Height: 5'5"
Weight: n/a
Hair: silver. shoulder length. it tends to be mostly in his face
Eyes: yellow/Green Eyes with pupils that resemble those of a cats
DoB: two years after Sephiroth's demise the first sight of creation is at the Northern Crater
PoB: The Northern Crater
Role: Leader of the Remnants
Family: Yazoo and Loz, calls Jenova Mother, and anyone with GeoStigma Brother or Sister
Allies: Yazoo and Loz
Enemies: Cloud, Tifa, Barrett, Cid, Vincent, Yuffie, RedXIII, Cait Sith, ShinRa, the Turks... etc.

He is obviously intelligent, but often comes across as childish. But with the release of Advent Children Complete, the spectrum of his almost obsessive personality is revealed. He is proven to be much more smart that shown before, and that he almost crafty in his thought patterns.
he is said to be the most like Sephiroth as a remnant. that is why he is able to take into sephiroths from
(spiller alert)
when Cloud defeats Sephiroth, he is not dead, but Kadaj is mortally wounded. when this happens his bond with Sephiroth is broken and he is free to be taken into the lifestream as his own being.