In the game, the real Sephiroth is in the end, and when you give him the black materia... The others you can see (before a battle against Jenova) is Jenova. She is the one who kill Aeris, (or Aerith, I don't remember) she's ever on your way...
So kadaj don't turn into Sephiroth, It is always Jenova! (like Sephiroth in the game... you see?)

=> sorry for my mistakes, I'm French so...^^