Cowboy Bebop is an anime series from the late 1990s that blends the genres of space opera, neo-noir and Western all into one story. Of the three genres explored, story-wise, it leans more heavily into neo-noir, with Jet Black fulfilling the hardboiled detective trope, Spike Spiegel playing the role of the reformed mobster trope and Faye Valentine being the femme fatale. Another character that heavily builds on neo-noir tropes is Julia, who falls somewhere between being a femme fatale and a Lost Lenore.
Julia is something of a mysterious figure in Cowboy Bebop. While she largely appears in flashbacks and in the series finale, she has a huge presence throughout the series. However, this begs an important question: why she is afforded such a high level of importance if she's barely present? The answer to that, of course, is that her absence is a major source of character motivation for Spike.
Little is known about Julia independently of the men in her life. One thing that's consistently clear throughout the series, however, is that she's tied to organized crime. Originally the lover of Vicious, Julia is consistently depicted as loving and nurturing, making the audience wonder how she got involved with someone like Vicious in the first place.
As the former right-hand man of crime boss Mao Yenrai of the Red Dragon Crime Syndicate, Vicious is a ruthless assassin. His level of empathy runs on empty, and he delights in the suffering of others, revealing a sadistic personality. When he was a member of the Red Dragon Syndicate, Spike often worked alongside Vicious as Mao's other right-hand man. However, unlike Vicious, Spike is empathetic toward others and has a strong moral compass (however much he might deny this).
At some point after a fatal gang fight left Spike severely wounded, Julia found him and nursed him back to health. Sometime later, Spike and Julia became romantically involved behind Vicious' back. The relationship made them reevaluate their lives, with both deciding to give themselves a fresh start somewhere else. Spike no longer wanted a life of organized crime and Julia most likely wanted to be free of Vicious. Tragically, this was not meant to be.
Vicious eventually found out about Spike and Julia's relationship and their plans to run away together. As a narcissistic individual, Vicious treated Spike and Julia's relationship as a threat to his reputation and a significant betrayal. In retribution, Vicious gave Julia an ultimatum: kill Spike at their agreed rendezvous and she would live, or he would kill both her and Spike. Julia chose to do neither and went into hiding instead.
Despite being heartbroken by Julia standing him up, Spike didn't hate her for it; he only wanted to know where she was and if she was okay. Whenever he heard any news about Julia or her whereabouts, he would pursue those leads in hopes of finding her.
While in hiding, Julia spent time on the Jupiter moon Callisto and attended the Rester House bar as a regular customer. She also met Gren -- a former associate of Vicious -- who played saxophone at the bar and lived nearby. Julia departed Callisto sometime before Vicious started using her name as a codename for smuggling Red Eye -- a deadly performance enhancement drug.
Julia eventually ended up back on Mars, where she was pursued by some of Vicious' henchmen. She was saved by Faye Valentine, who Julia knew to be a comrade of Spike. It was through Faye that Spike was able to reunite with Julia, who was now ready to pursue the life they both wanted years prior. Fate, however, reared its ugly head again, and Vicious succeeded in killing Julia before their plan could finally be realized. As the object of Spike's desires and fatal mistakes, Julia fulfills the femme fatale trope. As the lover Spike lost, she's also his Lost Lenore.