Isis holds the dual titles of Princess and High Priestess of Lower Egypt in ancient times. She is the daughter of Pharaoh Nefermaat and the elder half-sister of Pharaoh Memphis, a lineage granting her substantial religious and political power. Her actions are propelled by an intense romantic obsession with Memphis, driven by her desire to marry him and become Egypt's queen, believing this union essential for the nation's strength and prosperity—a conviction aligning with historical royal sibling marriage practices.

She perceives Carol Reed, a time-traveling American teenager beloved by Memphis, as both a romantic rival and a threat to Egypt. Isis relentlessly schemes to eliminate Carol, employing assassination attempts, imprisonment, and manipulation of political enemies. She collaborates with Emperor Ragash of Babylonia, agreeing to marry him if he kills Carol. Similarly, she frames Memphis for the murder of Mitamun, Princess of the Hittites, aiming to provoke war between Egypt and the Hittites and destabilize Memphis's reign. Her tactics include psychological manipulation, such as exploiting Carol's fear of water to trigger time-travel episodes that separate Carol from Memphis.

Her influence extends across ancient empires like Assyria, Babylonia, and the Hittites. She leverages political alliances and romantic promises to rulers like Emperor Algon of Assyria and Emperor Ragash, exchanging offers of marriage for their aid in removing Carol or undermining Memphis. Her deceitful actions frequently ignite international conflicts, including orchestrating the Hittite-Egypt war.

In the modern era, Isis infiltrates Carol's life by posing as a vulnerable woman searching for her missing brother. During this time, she enacts a curse sending Carol back to ancient Egypt. She indirectly causes the death of Carol's father, James Reed, using a cobra to kill him after he violates a royal tomb. These modern actions demonstrate her long-term commitment to disrupting Carol's lineage and influence across time periods.

Exhibiting no significant development or redemption, Isis remains consistently driven by her obsessions. Even after consequences like imprisonment, she persists in her schemes. Her static nature defines her role as a persistent antagonist creating ongoing conflict. Complex family dynamics, including her relationship with stepmother Tahrir—who murdered Nefermaat in an attempt to marry Memphis—provide context for her motivations rooted in familial betrayal and ambition.

Her name derives from the Egyptian goddess Isis, though she embodies none of the goddess's mythological attributes of healing or renewal. Instead, she represents a corrupted inversion, wielding spiritual authority and ritual practices like curses and sacrifices for personal vengeance and power consolidation.