Years before we meet him in Cowboy Bebop Gren fought in the Titan war. Although not much is known about it, it was there that he met Vicious and the last of his photos was taken. Vicious was not just Gren's comrade - he looked up to Vicious and perhaps even loved him. At one point Vicious was listening to a music box in the trenches, and Gren asked him if he would mind if he played that song (which Vicious called "Julia") on his sax when he went home. In a very profound moment for Gren, Vicious gave him the music box - seemingly strengthening their bond.
But when the war ended Gren didn't get to return home like his comrades; he was accused of being a spy and locked away in a military prison. The final nail in the coffin was when he heard that it was actually Vicious who testified against him. Despite that, he still held on to the music box that he had been given.
In the years he was imprisoned Gren nearly lost his mind from insomnia and started to take drugs. This lead to problems with his hormonal balance and as a side-effect he developed a pair of breasts, but he accepted his altered form and never let it become a point of bitterness.
Eventually Gren escaped from prison, and that is how he ended up with a bounty on his head. He went to Blue Crow, a rather seedy area of Callisto populated only by men, and started a new life playing the sax at a local club called Rester House.
It's not exactly clear when or how they met, but at one point during his time in Blue Crow Julia walked into his life. They became close friends and she would often go to the bar and listen to him play. Always requesting the same song, presumably "Julia," she'd sit in the corner and smile sadly. Julie told Gren all about Spike, and she was the first person he told about his past, and when Julia heard that the music box was a gift from Vicious she searched it and found a transmitter hidden within. In other words, it was Vicious who had been the spy, and not only had he testified against Gren he had also set him up to take the fall - knowledge which most certainly cemented Gren's feelings of betrayal. Then Julia left for reasons unknown.
It was at that time another woman walked into Gren's life; Faye Valentine had taken the money from the Bebop and ran, and ended up drowning her sorrows at Rester House. Gren approached her after his set finished, and although she thought at first he was coming on to her he assured her that he wasn't interested in women. Faye left in search of a fight and was almost given what she wanted by a local gang, but then met Gren for the second time when he came to her (unwanted) rescue.
After a successful escape he took Faye back to his apartment, where he gave her a warm drink and someone to tell all her problems to. When he went to have a shower Faye took the opportunity to look at all the pictures on his wall. At that point the phone started to ring and a horrified Faye heard Vicious leave a message for Gren as she could only stare in shock at the picture of Gren and Vicious together on Titan - once torn but now repaired with tape.
Filled with suspicion, Faye grabbed her gun and went into the bathroom to confront the showering Gren. However, when she pulled aside the shower curtain all she found were more questions when confronted with the fact that Gren had two things he obviously shouldn't.
After a very awkward moment for Faye, Gren put on a robe and then sat down to tell her everything, making her the second person since Julia to know his story. Faye was enraged that he was going to confront Vicious, knowing full well that he could easily die. But Gren wasn't afraid to die, and confessed that he probably brought her back to his apartment out of a desire for companionship before the end.
In true Faye style, she called him selfish and then grabbed her gun and opened fire, but her heart wasn't in it and he easily dodged the bullets. Gren managed to restrain her, and then left her handcuffed on his bed while he went off for his rendezvous with Vicious.
When the smoke fron the explosion cleared Vicious saw that it was Gren, and not a woman, who stood before him. Gren demanded to know why Vicious had betrayed him, reminding him of all they had been through and telling him he had always believed in him. Vicious only told him coldly that there is nothing to believe in.
Gren then opened fire on Vicious, but Lin, being loyal to Vicious and the Red Dragon, took a bullet for him and died. It was then that Spike arrived in the Swordfish, drawn there by the explosion. Spike had been desperately searching around Blue Crow for any trace of Julia. He had found out about the operation with the codename "Julia" and had ran off despite Jet's protests. The angry Jet told Spike he could only return to the Bebop if he collected the bounty on Gren's head, which was now advertised as double since it was due to expire soon.
Furious that Lin's life had been wasted for Vicious, Spike attacked but Vicious managed to grab the case of red eye and take off in his own ship. Soon he and Spike were fighting in the skies of Callisto.
Gren got into his own ship and joined in the battle where Vicious sent two missiles after him. One was destroyed by Spike, but the other hit its target and Gren's ship went down. Things seemed to be going in Vicious' favour, but then from within the case of red eye the melody from the music box could be heard; when it reached a certain point the bomb within exploded, damaging Vicious' ship and forcing him to retreat to the nearby Red Dragon ship.
Landing the Swordfish, Spike ran to Gren who was lying in the snow not far from his own damaged ship. Gently lifting him up, Spike begged him for information on Julia's whereabouts but Gren told him he knew nothing before being overcome with a coughing fit. Spike went to call for an ambulance when he saw that Gren was coughing up blood, but Gren stopped him and asked Spike to lift him into the pod of his ship so he could go to Titan. Spike protested, knowing Gren would never make it that far in his condition, but Gren was adamant in his desire to get as close to Titan as possible.
Spike finally gave in and carefully placed Gren back in his ship. It was then that Gren looked into Spike's mismatched eyes and realized that he was the Spike who Julia would always talk about. Spike asked Gren what Julia was doing in Blue Crow, but Gren could only tell him she would always request the same song and sit in the bar smiling sadly.
Spike towed Gren's ship behind the Swordfish out of Callisto's atmosphere before setting it adrift in space. Inside the pod Gren lay still, either dead or very nearly so, floating towards the place where so many people had suffered but where he had found happiness.