I read Plato’s various suggestions about the kind of thing that soul is as mainly practical, in the Kantian sense of being involved with ethics. As I read him, he does not intend to offer a consistent theory of the nature of soul.
There is strong textual basis for at least two rather different notions of “soul”, each of which I think Plato uses heuristically in a different kind of context. Soul might be a kind of essence that stands outside of time, or it might be a principle of life and cause of motion in time. Rather than trying to reconcile these at an ontological level as Plotinus and others did, I think it is more faithful to Plato to apply an Aristotelian semantic approach, and note that “soul” in Plato is said in more than one way. Plato is actually much more concerned with a more detailed level that is more directly involved with ethical considerations.
The Postmodern Peripatetic 