The character known as Cartaphilus is an immortal being who has lived for over two thousand years. The origin of his curse is rooted in an act committed during the crucifixion of Jesus, when he threw stones at him and was condemned to eternal life as punishment. This immortality is not a blessing but a cruel and painful affliction, as his body is in a constant state of decay, rotting away without ever allowing him to die. His physical appearance is largely hidden by bandages, and he has often been mistaken for a corpse due to his deteriorating condition.

Cartaphilus is not a singular entity but is fused with a young man named Joseph, a former gravedigger. Joseph, who was compassionate and sought to help the dead find peace, discovered the suffering immortal and took care of him out of pity. Unable to end Cartaphilus's pain or abandon him, Joseph made the extreme decision to merge with him, inheriting the curse of immortality in the process. This fusion resulted in a single, conflicted being who is often referred to as Joseph, as he hates being called Cartaphilus, which he calls a cursed name.

The personality of this fused being is a direct result of his unending suffering. The constant physical pain and the fear of his own body rotting drive his actions. He has lost the ability to feel empathy over the centuries, committing horrific acts and showing no qualms about inflicting misery on others if doing so helps him stave off his own deterioration or find an escape from his agony. He despises pain above all else and seeks a way to live without it. However, the fusion is not seamless; the original personality of the compassionate Joseph occasionally surfaces, leading to moments of confusion, memory loss, and even brief flashes of his former kindness, highlighting the internal conflict between the two souls sharing one body.

In the story, Cartaphilus serves as a recurring antagonist whose actions are largely motivated by the desperate search for a cure or a means to end his endless life. His methods are ruthless and include becoming a master alchemist specializing in creating chimeras and conducting cruel experiments. To maintain his decaying form, he requires a constant supply of replacement body parts. He sees Chise Hatori, the protagonist, as a key to solving his predicament, and he orchestrates a bargain to exchange parts of their bodies in an attempt to transfer his curse or gain a more sustainable form. He also demonstrates a willingness to use others as pawns, such as taking control of the young girl Stella Barklem's body and using her as a hostage to manipulate Chise.

His key relationships are defined by his antagonism and desperation. He is a direct foil to Chise, as both are cursed, but while Chise seeks to learn, grow, and connect with others, Cartaphilus has been consumed by his suffering, leading him down a path of cruelty and isolation. He also has a tense and antagonistic relationship with Elias Ainsworth, the Magus' Bride of the title, and seems to know more about Elias than most other characters. After a climactic battle, he is defeated and concedes, choosing to remain dormant for roughly a century in Elias's well.

Cartaphilus possesses a wide array of notable abilities derived from his curse, his alchemical knowledge, and his origins as a necromancer. His primary ability is his curse of immortality, which prevents him from dying from any wound or illness but does nothing to stop his body from constantly rotting and suffering harm. He can transform parts of his body into monstrous, shapeshifting forms, such as giant claws or tentacles. As a master sorcerer or alchemist, he can create powerful chimeras. His necromantic heritage allows him to communicate with the dead and summon the wraiths of those he has killed. He is also capable of body possession, as demonstrated when he took control of Stella, and can summon a tide of giant centipedes to attack his enemies.