Cross Marian is a prominent figure in the Black Order, holding the distinguished rank of one of its five Generals, a title reserved for its most powerful exorcists. He is perhaps best known for being the master and teacher of the series' main character, Allen Walker. A man shrouded in mystery, his past includes work as a scientist before he became an exorcist, and he has a habit of disappearing from the Order's headquarters for years at a time, citing a deep dislike for the place. In terms of appearance, Cross is a remarkably tall man, standing at 195 centimeters, with long, vibrant red hair and a distinctive white mask that covers the right side of his face, often compared to the style of the Phantom of the Opera. He typically wears an explorer's outfit complete with a wide-brimmed hat and carries his primary weapon in a holster on his leg.

The personality of Cross Marian is famously rough and complex. He is a gruff, demanding individual who is incredibly hard to please, often referring to his own apprentice as an "idiot disciple." On the surface, he appears to be a hedonist, driven by a love for expensive wine, gambling, and beautiful women. He is a notorious spendthrift who lives well beyond his means, and rather than pay his own debts, he is infamous for leaving the bills for others to settle, most frequently his student Allen. He also has a known aversion to dirty or unclean things, although this standard seems to be flexible when it comes to the company of attractive women. Despite this selfish and sometimes cruel exterior, there are clear indications that he cares deeply for those close to him. His harsh training methods hide a genuine desire to prepare Allen for the harsh realities of their world, and he has been shown to have a softer, more protective side, particularly towards women like Lenalee Lee or his former lover, Anita.

Within the story, Cross plays a critical and secretive role. He has been out of contact with the Black Order for four years before the main events of the series, choosing to wander on his own. It is eventually revealed that he is a key figure in the central conflict, having been a collaborator and supporter of the Fourteenth Noah, a mysterious individual who was killed by the Millennium Earl. At the Fourteenth's request, Cross took on the responsibility of watching over his brother, Mana Walker, who would later become Allen's adoptive father. This connection places him at the very heart of the mysteries surrounding Allen's true nature. His knowledge and power make him one of the few people the Millennium Earl is genuinely concerned about. A significant part of his character development involves his relationship with Allen, which evolves from a purely authoritative and abusive master-disciple dynamic into something more tragic and paternal. He is the one who ultimately reveals to Allen that he is the host of the Fourteenth Noah's memories, a truth that sets his apprentice on a new, perilous path. Later in the series, Cross is found grievously wounded, and after disappearing, he is presumed dead by the Order. However, his consciousness later appears within Allen's mind to provide guidance, leaving his ultimate fate ambiguous.

Cross Marian is exceptional among exorcists for his ability to simultaneously wield two different types of Innocence, the anti-Akuma weapons used by the Order. His primary weapon is an equipment-type named Judgement, which takes the form of a large, ornate revolver. This weapon is incredibly powerful, capable of firing high-caliber bullets that will track their target unerringly. Its special attack is the Arrow of Original Sin, a devastating blast of energy. He can also increase Judgement's purging level, amplifying its power to a massive degree. His second weapon is a parasitic-type called the Grave of Maria. This is a unique and forbidden tool, as it does not originally belong to him. It takes the form of a large, chained coffin that contains the reanimated corpse of a female exorcist named Maria. Using his knowledge of magic, Cross is able to control Maria's body and use her Innocence in combat. This ability grants him powerful defensive techniques, such as the Magdala Curtain, which creates illusions to conceal allies, and Carte Garde, a potent ability that allows him to take direct control of an enemy's brain and manipulate their movements like a puppet. In addition to these abilities, Cross is also the only known exorcist capable of modifying and converting Akuma, turning these demonic weapons into soldiers that will fight on his own side.