Gren was once a soldier for the war on Titan, and appears in the two-part episodes of 'Jupiter Jazz'. On Titan he fought beside Vicious, who he admired and found encouragement in, during the war. After the war, Gren came back hoping to be a jazz musician, but that plan was cut short when he was arrested on the pretense of being a spy. In prison, Gren heard that Vicious testified against him; this and the isolation drove him mad. The prison used prisoners for drug experiments, and he was forced to become a test subject. As a result, the drugs severely imbalanced his hormones, and caused him to develop a feminine figure, including breasts.

Finally, after his body had been transformed and his sanity shattered, he managed to escape from prison. After his escape from jail, Gren worked as a saxophone player at Rester House, a bar in a sector called 'The Blue Crow', which is located on one of Jupiter's moons, Callisto. He met Julia there and found out from her how Vicious betrayed him. Two years later, Faye met Gren. In an act of kindness, Gren 'rescued' Faye from a fight she caused and allowed her to come to his apartment. While Faye was there, Vicious called, alarming her and raising suspicions about the musician. Intruding on him while showering, Faye discovered Gren's secret. Gren explained how he became what he was, and told her that he was going to see if Vicious really framed him. When Faye said he'd be killed in the process, he replied that death didn't frighten him, and that he would not live longer anyway.

Disguising himself as a woman, Gren met the Syndicate member, along with Lin, who went to protect Vicious. Exchanging Red Eye for Titan Opal, Gren suspected a trap. He shot it open, setting off the explosive, and then revealed who he was, finally confronting Vicious about what happened when he returned from Titan. In the ensuing battle, Lin died to protect Vicious. Meanwhile, Spike arrived and began to attack Vicious as he fled. Gren had planted the music box in the bag of Red Eye, rigged with an explosive, which damaged Vicious' ship. However, his own ship was severely damaged in the process. Spike, wanting to find out more about Julia, lands next to Gren's crashed ship, and finds Gren lying in the snow. Spike rushes to Gren's side, and as he dies, Gren mentions something that Julia said about Spike. As Gren's last request, Spike helped him back into his ship and launched him toward Titan.