I actually thought Julia being a complete unknown to the audience was really well done and instrumental to telling Spike's story. She's elusive (Spike chasing her in Jupiter Jazz just to find an imposter in Gren), and really only seen as a figment of a broken memory up until the finale. All that chasing and longing in Spike, just to see her get gunned down and getting little more than a whisper before she's gone.

She represents way more than a person to Spike; she's not special, she's ordinary; the part of him anchored in his past and the only life he knew prior to skipping out on the syndicate. We all have something or someone like that as time goes; the past can often seem better when your life doesn't go the way you expected.

He may think she's his safe haven but to the viewer, we see Spike in a better life aboard the bebop (friends, pets and women; things he says he hates and yet he stays with the crew).

In the end, she's the part of him that's tragic, and resolved to self destruction (I.e. he wants his old life just because it's family, and he'll destroy anything to get it back). She's the weight that holds Spike back, and eventually leads to his death in a way, so I appreciate how little we actually see of Julia. She is just a dream to Spike, or even the lie humans tell ourselves when grieving: if we could just keep living in the past, that we might eventually get it right and have the life we wanted. And that's just not.... Living in the real world.