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 was 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.
As time went on however, Marrow's relationship with the other Ace-Ops became strained after he stated that he wouldn't go through with Ironwood's plan anymore and eventually used his Semblance to restrain his former teammates.
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.
In "Divide", he is shown mourning Clover's death alongside his teammates.
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.
In "War", Harriet states that Marrow was a replacement for their old teammate Tortuga, implying that she only sees Marrow as a replacement rather than a friend.
Marrow and Elm seem to share a form of rivalry one another, where Marrow claimed he is "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 assured Elm's views on Marrow's loud mouth, walking away whilst laughing at him. Later, Vine joined Marrow when introducing Teams RWBY and JNPR to the Huntsmen board. Later, Marrow, Elm, and Vine helped the new Huntsmen team train.
During "The Final Word", Marrow was saddened when Vine was forced to sacrifice his life to save his friends lives.
Before Winter was promoted to the leader of the Ace-Ops, Marrow and Winter didn't have much interaction with each other.
After she was promoted however, Marrow was originally willing to follow her orders. During the Battle of Atlas, their relationship seemingly became strained when Marrow asked Winter if she was willing to bomb her sister's friends. While Winter initially responded with a cold "yes" like general Ironwood would have done, she decided to make the choice of turning around and addressed Marrow with more warmth to show him that she wasn't thinking like Ironwood. Marrow's betrayal against the military was delayed because of this kindness.
After Marrow stated his decision to stop helping Ironwood, Winter seemingly arrested him for his betrayal. However, it was later revealed that Winter only acted like she was arresting Marrow in order to save him from being killed by Ironwood despite his complaints about being tackled and also realized that Ironwood needed to be stopped.
Ironwood was shown to have been impressed by Marrow's abilities and skills as a Huntsman and unlike a majority of the Atlas Elite, did not discriminate him for being a Faunus and recruited him to join the Ace-Ops regardless.
Marrow in turn, like his teammates, has shown to have a great deal of faith and loyalty in the General in return and follows his orders without question. This is seen where he had defended the General's action numerous times against the criticism of others despite the increasingly suspicious nature of the General's actions and remained steadfast to his devotion to Ironwood.
His loyalty is seen where he complied with the General's orders to abandon Mantle in favor of saving who they could in Atlas and later followed his orders to capture and arrest Team RWBY despite becoming close with them.
However, he was visibly shocked when Ironwood killed Councilman Sleet in "Divide", but even after he continues to follow his orders, as shown when he and rest of the Ace-Ops were sent to capture Penny in "Strings".
As shown and stated by Lie Ren in "War", Marrow himself is secretly conflicted by the new orders Ironwood has given him and the rest of the Ace-Ops and he himself is no longer fully on board with what the General is trying to do and wants nothing to do with it anymore. This is proven in "Risk", where Marrow speaks out against Ironwood's decision to bomb Mantle to force Penny to cooperate and decides to quit the Ace-Ops, seeing the rank he wore as a collar instead of an honor. After his outburst in front of Ironwood however, Marrow did not anticipate that his reaction to it would be so extreme as he slowly pulled his gun towards his head as he took a while to process what was happening before Winter saved Marrow from being killed by pretending to arrest him.
As seen during the events of Volume 7, Penny and Marrow got along enough during their time working together in Atlas doing various assignments for Ironwood and protecting the people of Mantle and Atlas.
As shown in "A Night Off", Marrow had somewhat of friendly junior-senior relationship with Penny during their work as a Huntsmen where he tries to keep Penny focused on their assignment of keeping watch at Robyn's victory party and insure no incidents. Penny herself was shown to be friendly enough with Marrow to work together with him and find Ruby's jokes made at his expense funny.
Later after Tyrian Callows massacred numerous people at the party, Marrow was shown to be horrified and disbelieving that Penny could do such a thing and protected her from being mobbed by the angry citizens and supporters of Robyn and later used his Semblance to buy time for her to escape.
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 "Amity", Marrow was seen smiling during Ruby's broadcast before stopping when Elm looks at him.
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 "Witch", it is shown that Marrow still does not remember Jaune's name, referring to him as "Juan" once again.
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.
As seen in "Witch", despite them now being on opposite sides, Marrow has shown to still have a strong care for Jaune, this is seen where he showcased clear worry for Jaune and his group when they entered the Monstra and attempted to dissuade Winter from immediately placing the bomb into the Monstra and give Jaune and his friends a longer time frame to get Oscar. While in the battlefield fighting Salem Army, Marrow showed clear worry for Jaune and prayed that he and his friends had already escaped from the Monstra.
- RWBY Volume 8 Directors' and Writers' Commentary Chapter 9
- RWBY Volume 8 Directors' and Writers' Commentary Chapter 11