Marrow was a student at Atlas, and graduated as a member of the military.
Marrow is the newest addition of the Ace Operatives, Atlas' elite fighting force. As a result, he is one of James Ironwood's most trusted individuals, with knowledge of Salem and her group, as well as the Relics and the Maidens. He later defends Ironwood's actions to the Happy Huntresses in "Sparks".
Marrow is the newest member of the Ace-Ops team. As the rookie, he is often the butt of the Ace-Ops' jokes, having to be 'babysat' by the older members.
After their mission in the Schnee Dust Company Mine #2, Marrow is seen imitating Clover's posture and strut, looking at him for a reference.
Later, Clover chuckles at Marrow after seeing him dramatically tell an Atlas Soldier about the mission in the mine.
Marrow and Harriet seem to share a bond, where Harriet often teases Marrow like a sarcastic older sister, pointing out she has to babysit him during the Ace-Ops' missions, to Marrow's annoyance. Later, when Marrow claimed he was "zero liability", Harriet can be seen laughing while walking away as Elm points out his talkativeness was a liability.
Marrow and Harriet pair up with one another during missions, as their Semblances complement each other well, where Marrow freezes targets to set up a take-down for Harriet.
In "With Friends Like These", Harriet berates Marrow for holding back when attempting to stop Team RWBY, which he retaliates with the fact she was being excessive, and he wasn't trying to kill RWBY. Harriet ignores his points, and continues to fight at an overly aggressive pace.
Marrow and Elm seem to share a form of rivalry one another, where Marrow claimed he "all asset and zero liability", Elm chastised him, claiming they had to put up with his loud mouth. Marrow reacts in frustration as she walks away, laughing.
Vine assures Elm's views on Marrow's loud mouth, walking away whilst laughing at him. Later, Vine joins Marrow when introducing Teams RWBY and JNPR to the Huntsmen board. Later, Marrow, Elm, and Vine help the new Huntsmen team train.
While originally showing annoyance over Team RWBY's more childish behavior, Marrow's kindness and fondness for the girls is fully shown in "With Friends Like These" where he is noticeably shown to have the most hesitance to fight, having attempted to stop his teammates and Team RWBY from fighting each other. Even after being berated by Harriet, Marrow still refused to use excessive force to hurt them and stated he was trying to arrest them, not kill them, showing how much he has come to care for Team RWBY during their time in Atlas working together.
In "Pomp and Circumstance", Ruby showed admiration to Marrow and the other members of the Ace Ops, being awed by the fact that they took down the Geist from "Ace Operatives", without using any kind of tactics.
In "Ace Operatives", Marrow explains to Weiss and Blake the societal issues with the Faunus in Atlas. He also tells her despite the abuse his species faces, he's not looking to solve those issues.
In "Out in the Open", Marrow and Weiss defend Mantle's citizens alongside one another. Weiss kills a Beowolf attacking Marrow with her Summon, nearly cutting at and startling Marrow in the process. Marrow's reaction causes her to giggle.
During their fight in "With Friends Like These", Marrow tries and fails to talk Weiss into turning herself in, telling her that while he knows she's used to getting what she wants, the fight was pointless. Due to his hesitance to fight, Weiss defeats Marrow and later binds him to his Gravity Bolas.
In "Ace Operatives", Marrow explains to Weiss and Blake the societal issues with the Faunus in Atlas. He also tells her despite the abuse his species faces, he's not looking to solve those issues. Marrow trusts Blake enough to scout the collapsed mines alone without backup.
Later, in "Sparks", he asks Blake and Yang if they should pair off with other team members, noting they only ever worked with each other.
Marrow asks Blake and Yang if they should pair off with other team members, noting they only ever worked with each other.
Later, in "Sparks", he asks Yang if she should pair off with other team members, noting she only ever worked with Blake. This angers Yang, to the point of making Marrow nervous.
In "Pomp and Circumstance", Jaune showed admiration to Marrow and the other members of the Ace-Ops, being awed by the fact that they took down the Geist from "Ace Operatives", without using any kind of tactics. Later, after viewing the mission board, Marrow seems to not really care about to remembering Jaune's name, such as calling him "Juan", "Jwan" and "Jim" (With Jaune awkwardly commenting that the names were close enough to his own).
In "Sparks", Jaune was annoyed when Marrow giving him a pleading stare while waggling his tail; pleading silently to the leader of Team JNPR to give him his coffee. Even so, Jaune still gave the dog Faunus his coffee with a frown on his face. Later, Jaune and Marrow seem to be getting along with each other a little better, and the next time Jaune brought with him two mugs of coffee and hands Marrow the other one. Smiling to each other, they clink their mugs together.
Marrow displays a disliking towards the Happy Huntresses, alluding to the fact he'd have no qualms towards arresting them if they did anything that seemed remotely illegal, hinting at their borderline vigilante activities.