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Colt is a physically fit young man with light eyes and short light hair. Like his fellow warrior cadets, he wears the standard Marleyan soldier uniform consisting of a light jacket and pants, supply packs on the hip, suspender straps, a hard hat with a stripe around it, and tall combat boots. As an Eldian his uniform is different from that of the Marleyans in that the shirt collar has a small badge with the Eldian star instead of a small clip and the hat has a white stripe with the Eldian star on it instead of a pure black stripe. As is law in Marley, Colt also wears the Eldian armband as all his family and ancestors residing there do as well.
Colt is a dedicated and brave soldier, risking his life to go retrieve his wounded brother Falco after he comes under enemy fire. He shows great care for his brother as he gives him his hard hat as they flee and makes sure his wounds are treated when they return to the hiding trench. While caring and brave, Colt is also a tactical soldier who works with his Commander to plan their attacks, despite his superior officers looking down on him for being an Eldian. Colt always appears to have a guess of what moves might be made in advance by the enemy and tries to strategize to use their forces and the Power of the Titans to best take out the threat without losing lives.
However, he can lack discretion in times of leisure, having asked Pieck to share her alcoholic drink, despite being very bad at handling his liquor. This has happened often enough that most of his comrades and even superiors are aware of this trait.
In 832, disgrace was brought upon the Grice family when his uncle's involvement with the Eldia Restorationists came to light. To prove his family's innocence, Colt was forced to pledge service to the Marleyan military as a warrior cadet, eventually getting chosen to be the next inheritor of the Beast Titan power.
During the Marleyan war with the Mid-East Allied Forces, Colt is present among 800+ other Eldians fighting on behalf of Marley. However, he is valued as a warrior cadet with the potential to inherit the Beast Titan power. During the fight, Falco and some of his comrades are hit by enemy fire, killing some but sparing Falco and leaving him wounded. Colt appears to save him and puts his hard hat on Falco's head before carrying him on his back while avoiding enemy fire. When Colt returns to the hiding trench with his wounded brother, their fellow cadets tend to his wounds. Commander Magath shows concern for Colt's well being but then demeans him when he reports that they are unable to dig any further due to enemy fire.
Colt voices concerns over the enemies' artillery, saying they should let Porco Galliard and Pieck handle the enemy gunmen, but Magath refuses. The appearance of the anti-Titan artillery leads Magath to order his 800 Eldian warriors into battle to take it out, but before this can happen the plan is canceled in favor of Gabi's idea to solo attack it with a bomb. Colt watches as Gabi executes her attack and succeeds. When she is under fire from one remaining survivor, Falco rushes out of the trench to save her, much to Colts concern. Luckily, the two are both saved by the transformation of Galliard and the last enemy gunmen is taken out.
After the war, during the meeting of the high-class officers, Colt is seen behind Zeke Yeager, who refers to Colt when he talks about the resume of Paradis Island Operation. After the meeting, Colt is seen talking to Zeke and discusses his Beast Titan's Power. Just as Zeke is about to unveil his secret, Commander Magath appears and stops the current conversation as Colt lights the cigarette for his superior. Later, he is seen drunk in the train as he praises Gabi for her bravery of taking the Armored Train down in place of 800 cadets.
Colt is later summoned to the warriors' headquarters, where he attends a meeting with the current warriors to discuss the dangerous position Marley's weakened state puts the Eldian people in, and Zeke's plan to resume the attack on Paradis.
After a month has passed, he attends a strategic meeting to determine how best to strike at Paradis. While there, he bares witness to his brother beating Gabi in a sprinting exercise. However, he claims it is too late for Falco to overtake her, for which Zeke mildly berates him. The two comrades are later seen playing catch-ball, preparing Colt for his role as the next Beast Titan.
- Falco Grice - Colt is Falco's older brother and cares deeply for him, as he threw himself into the battlefield to rescue his injured brother, even giving him his hardhat as they fled. After he witnesses Falco surpassing Gabi in a footrace, Colt expresses concern over the possibility that Falco will become a warrior too.
- Zeke Yeager - As a candidate to inherit the Beast Titan's power from Zeke, Colt greatly admires Zeke and his Beast Titan's unique ability to create and control Titans. The two have a mentor/protegee relationship with Colt standing by his side in important meetings and even playing catch together, probably as a form of training for the Beast Titan's long range attacks.
- Theo Magath - While Commander Magath may look down on Colt for being an Eldian, he values him as a soldier and showed concern for his well-being when he risked his life to save his brother. Colt appears to show some margin of respect for this man, as he attempts to work through strategies with him and is not above giving his opinion of the best move.
- He shares a birthday with the Trost District citizen.