And that Scar is immortal. So perhaps that's how it manifests in characters that are beyond death. And Scar is a conduit of the black--goop, energy, whatever, and so perhaps Donna will be too. Their version of the Guardians.
I have to say, reading the end of this one just now, this does sound like Terra. Interestingly, the last time I read the Titans was, I think, 1987 or so, and picking this up it feels like almost nothing has happened since. Make of that what you will.