It is Jenova that is going around in the form of Sephiroth. Sometimes this body controlled by Sephiroth (when "Sephiroth" recognizes Cloud) and other times controlled by Jenova (when "Sephiroth" doesn't even know Cloud). Jenova is not being carried by Sephiroth because Sephiroth is in the Nothern Crater crystallized. The reason you fight a Jenova part (Jenova-Birth, Jenova Birth, Jenova-Life) everytime you encounter Sephiroth (except for the end) is because Sephiroth is not actually there. So Jenova leaves parts of itself. Cloud and the rest of the crew thinks Sephiroth is actually physically walking around. But that is because they do not know thatit is Jenova's body taking the shape of Sephiroth and not actually Sephiroth. Sorry but that is as clear as I can explain it. If SoS was here he would be able to explain it very well.