The character Gren, whose full name is Grencia Mars Elijah Guo Eckener, is a former soldier who fought in the Titan War, a conflict that forms a significant part of his tragic backstory. Following the war, he was arrested and tried as a spy by a military court under the belief that a comrade from the war, Vicious, had testified against him. The psychological torment of this betrayal drove him to a breaking point, leading prison authorities to administer an experimental drug intended to calm him. While the drug was addictive, its side effects were severe, drastically altering his hormonal balance and causing him to develop a more feminine figure, including breasts. He eventually escaped from prison and went into hiding.

In terms of personality, Gren is portrayed as a deeply melancholic and solitary individual, shaped by the immense loss and suffering he has endured. Despite his bitterness and trauma, he is not without kindness or a sense of sincerity, which is most evident in his interactions with others who are also lost. He is introspective and cautious, yet carries a quiet intensity, particularly when the subject of his past arises. His life on the run has made him self-sufficient and pragmatic, but he has not lost his capacity for empathy, offering understanding to those in pain while guarding his own closely. He found a new, albeit fragile, purpose as a saxophone player at a bar called the Rester House in the town of Blue Crow on the icy moon of Callisto. The music he plays, especially the song "Space Lion" from a music box given to him by Vicious, serves as a constant, painful reminder of the friend who ultimately betrayed him.

Motivation is the central engine of Gren's appearance in the story. His entire scheme to lure Vicious to Callisto for a fake Red Eye drug deal is driven by a singular need to confront him face-to-face. He is not seeking revenge in a simple, violent sense, but rather an answer to a profound personal question: why Vicious, a man he admired and tried to befriend during the horrors of war, would testify against him and destroy his life. This need for an explanation, for some kind of closure or meaning behind his immense suffering, is what propels him forward, even when he acknowledges that he has little time left to live.

Gren's role in the overarching narrative is that of a tragic foil, primarily within the two-part episode "Jupiter Jazz". His story serves as a dark mirror to the series' main characters, particularly Spike Spiegel and Faye Valentine. Like Spike, he is a person haunted by a past tied to the villainous Vicious and the syndicate, unable to let go of a connection that defines him. Like Faye, he is a drifter who has lost their former identity and body to forces beyond their control, finding an unexpected moment of connection and understanding with her when she discovers his secret. His confrontation with Vicious provides a crucial exploration of themes like betrayal, loneliness, and the refusal to form bonds, directly contrasting Vicious's cold declaration of needing no one with the desperate lengths to which Gren goes for an answer.

The most significant relationship in Gren's life is with Vicious. He looked up to him as a soldier and tried to forge a comradely bond, keeping the music box Vicious gave him as a treasured token. This admiration curdled into an obsession after the betrayal, and his final confrontation is a desperate attempt to understand the man he once respected. His friendship with Julia, who frequented the bar where he played, is another key relationship. She became a confidante, and he trusted her enough to share his story and the secret of the music box. It was she who suggested he open the box, revealing the solar transmitter inside and confirming his suspicions about Vicious. His brief but poignant interaction with Faye Valentine offers a moment of mutual recognition between two damaged individuals, and his final moments are spent with Spike, whom he correctly identifies as the man Julia spoke of, asking for a final kindness.

While Gren does not undergo a traditional arc of change, his development is found in his final release. He moves from a state of passive hiding, nursing his bitterness while playing jazz in a secluded town, to active pursuit of the truth. Even after failing to kill Vicious and suffering fatal injuries, he achieves a form of resolution. His final request is not for revenge or rescue, but to have his ship towed into space and set on a course for Titan, the site of his wartime service, so that he might die on the way there. In his death, he finds a kind of peace, smiling as he drifts toward the place that represents his past, finally letting go of the weight he has been carrying.

Gren has several notable abilities stemming from his dual life as a soldier and a musician. He is an accomplished saxophone player, whose music is a central expression of his soul and his pain. His military training has left him with combat skills, including proficiency with firearms, which he demonstrates during his shootout with Vicious and his ability to disarm Faye Valentine. This combination of artistic sensitivity and learned lethality makes him a unique and compelling figure in the series' rogues' gallery.