After the war, he was tried as a spy and imprisoned. He later moved to Callisto and played saxophone in the Blue Crow while also becoming involved in the illegal Red Eye trade. He was killed after confronting Vicious (Gren's war comrade) during a drug trade he had arranged .
In the Titan War
Little is known about Grencia Mars' past. When he deployed during the Titan War, he was stationed in the same company as Vicious. Gren tried to befriend him, joining him during his usual isolation between battles. Hearing a music box he was playing, Gren asked him about the tune it played to start a conversation. Vicious let him keep the trinket, and stood up to leave, but not before startling him by killing a scorpion near his head. Unknown to Gren, this music box contained a solar transmitter for tracking him. During their time afterwards, Gren grew to admire Vicious as a soldier.
After the war, Gren was tried as a spy by a military court and sent to prison. During that time, he heard of a rumor that Vicious was the one who testified against him. Gren had a lot of trouble with this, so much so that he was given an experimental drug to calm down. Unfortunately, the drug had side effects – it increased his estrogen levels, causing him to grow breasts. Eventually, he escaped from prison.
Gren hid out in Blue Crow, where he lived at the Orlando Apartments – a blue building in a town completely devoid of women. He started playing the saxophone regularly at the Rester House. He met Julia when she first visited the bar in 2069, a rare sight in the town. She proceeded to visit almost every night for a month, and they became friends. She would often request for him to play the same song: Julia. Julia also talked to Gren about Spike often, noting the colors of his eyes among other things. Gren told her about himself and showed her the music box he got from Vicious. When she heard who it was from, she told him to open it up knowing that the solar transmitter would be there. Gren became angry again and decided to confront Vicious to learn why his war comrade testified against him but needed to bide his time to make a plan.
Gren decided to lure Vicious to Callisto for a Red Eye drug trade. He put his plan in motion in 2071, setting it up using Julia's name to draw Vicious out. The night before the deal, he met Faye Valentine and saw how she was dealing with her own problems. He brought her to his apartment thinking she was going to get herself into trouble. He was confronted whilst showering by Faye when she realized he knew Vicious . Gren then explained his past to her after she subsequently discovered his intersex qualities. She ended up trying to shoot him for having ties to Vicious (an enemy of hers), but Gren managed to disarm Faye and left her handcuffed to his bed while he carried out his plan .
Disguised as a woman similar to Julia in a cloak, Gren met Vicious on the rooftop of a skyscraper with a bag of Red Eye and the music box hidden inside. He let Lin check the Red Eye, but, when he started walking back, Gren then mentioned the scorpion that Vicious killed the day he gave Gren the box, revealing his identity. Guessing the suitcase had a bomb in it, Gren kicked it over and shot it, making it explode. Vicious ran and was uninjured, but Lin was hurt. Gren confronted Vicious about his behavior, but Vicious simply replied that there is no need to believe in anything. Angry, Gren then shot at Vicious, but Lin shielded him, dying in the process. They both ran to their ships, and Vicious shot Gren's zipcraft, forcing it to the ground. Fortunately, the Red Eye bag Vicious took to his ship exploded, forcing him to retreat and not finish Gren off. However, Gren suffered fatal internal injuries from the battle. In his last moments, he met Spike Spiegel and realized that he was the person Julia was often talking about. Despite Spike's questioning, Gren didn't have the answers to where Julia was or what happened to her. He then requested Spike power up his engines and tow his ship into orbit so he could drift to – and rest in peace on – Titan, in which he died on the journey .
- Kawamoto Toshihiro, character designer for the series, stated that the most challenging character to design was Gren, as Gren was originally modeled after Brad Pitt, but Kawamoto had trouble picturing "Brad Pitt with breasts."
- Gren appears to have gynecomastia (males growing breast tissue), which is not uncommon in men taking steroids and other drugs targeting hormonal influences, after being forced to take experimental drugs while in prison.
- Gren's style of playing the sax is highly inspired by The Crying Game's sax scene.
- Gren reveals that he's gay or asexual when he told Faye that being in a sexual relationship with a woman is not his style.