Marquis Janus emerges as the primary antagonist in *Great Mazinger*, succeeding the deceased Archduke Gorgon as chief adviser to Minister Argos. Her striking visage presents two distinct faces: one a beautiful young woman with pale skin, the other a demonic hag boasting green, wrinkled skin and wild hair. Both faces share unsettling yellow sclera and black irises. She typically wears a long dark red robe and sports horn-shaped purple hair, often seen with a black cat perched on her shoulder and wielding a white staff. For human disguises, she adopts loose hair and attire such as a green dress featuring yellow bands and a diamond-shaped chest opening.

Unwaveringly loyal to the Mycenae Empire, Janus exhibits a cruel and sadistic nature towards her enemies. While respecting her commanders, she actively competes with rivals like the Great General of Darkness, frequently mocking his sense of honor. She favors psychological tactics, aiming to break her opponents' spirits before engaging them physically.

Janus commands formidable supernatural abilities including flight, teleportation, invisibility, and energy beams projected from her eyes or staff. Though appearing as witchcraft, these powers likely originate from advanced Mycenae technology, evidenced by her use of electronic spikes for mind control. She pilots the personal robot *Janus Koshaku* and leads the Catloo Corps. Her strategic acumen surpasses Gorgon's, enabling her to identify and ruthlessly exploit weaknesses in the Great Mazinger and the Science Fortress Laboratory. She also possesses significant physical strength, demonstrated by tearing a person vertically in half, and is skilled in disguise, impersonating figures like Sayaka Yumi or using a human form to infiltrate enemy strongholds.

In the anime continuity, Janus orchestrates relentless assaults against the protagonists following Gorgon's death, overwhelming them repeatedly until their narrow victories. Her final battle concludes with her robot crashing into the computers of the *Demonika* fortress, triggering an explosion that kills her. Conversely, the Gosaku Ōta manga adaptation presents a divergent path: Janus murders Misato before Tetsuya Tsurugi to demoralize the heroes. Later, after the Great Marshall of Hell brands them outlaws, she lures them to a volcanic hideout. While her forces initially stalemate the Great Mazinger and its allies, a volcanic eruption engulfs Janus and her troops during the hideout's collapse.

Her name derives from Janus, the Roman god of beginnings and transitions, reflecting her two-faced nature.