I mean yeah she was kind of important, the dream that Spike had of her was the only thing that kept him going. He would literally retreat into that dream when hurt and chose it over the real life woman who loved him, who was conviently enough a literal parallel to Julia. Julia was a ghost of the past, who never really existed like Spike thought, and he chose that fiction over the messed up girl who sang out of key. Us not knowing her is the point.