Reddit user blueskies8484 theorized that the Monster's "sister" was Pandora, a figure from Greek mythology. Pandora famously opened a box that contained all the world's ills, releasing horror and sorrow of every sort into the universe. This user puts forth the idea that the Monster is actually one of the "pure elements" that Pandora released from the box; it wouldn't make them siblings, but it would connect the two. If the Monster feels indebted to Pandora for releasing it, it may look upon her with a sibling's affection and be angered at her death.

Redditor ChaoticNeutralPrime pointed out that the Monster said he and Bacchus had the same parents. In that case, it stands to reason that one of Bacchus' sisters could also be the Monster's sister. The identity of Bacchus' mother is debatable, but one theory is that his mother is Persephone. She has a daughter named Melinoë, who is a minor goddess or nymph connected to nightmares and magic. Since Persephone and her problem children have already been recurring parts of the show, this could bring it all full circle.

A few fans have theorized that the Monster is Cronus, a Titan who fathered the majority of the Greek pantheon. Cronus had a sister-wife named Rhea. Titans aren't technically gods, but powerful beings who existed before Zeus or Hera ever ascended to Olympus. Cronus and Rhea actually had some conflict, so if that's who the Monster and his sister are, it's unclear why he'd want to revive her. In Greek mythology, Cronus consumed all of his children with Rhea so they couldn't overthrow him — until she hid Zeus from him, which led to the exact situation Cronus had feared. Maybe the Monster only wants to get her back for causing his downfall.

Redditor CommonSensibility looks to Egyptian mythology for an answer instead of Greek. Osiris was an Egyptian god who was betrayed and dismembered by his brother Set before his sister-slash-wife Isis put him back together. This mirrors the Monster's story in obvious ways, with his body being taken from him in an act of godly betrayal. Osiris seeking to restore Isis would flip the story somewhat, but it would echo back to the references to Egypt scattered throughout the season. A mummy had important advice about the Monster, and Julia found answers about its body in hieroglyphics. Perhaps that's because its origin is Egyptian.

Reddit user Galgos thought perhaps the Monster and his sister were actually good and the gods who punished them were the real villains. They thought the Monster siblings could be Ares and his sister Enyo (both gods of war). They were targeted by Aegnus so he could take Enyo's power before killing her and putting her brother in Castle Blackspire. It's a possibility, though the ice cream man who offered the Monster sprinkles in Episode 1 might have some doubts about how good he is.

Iris was one of the gods killed by the Monster, which Redditor monroeet thought was an important detail. The family trees of gods can make your head spin, but Iris had an uncle and aunt named Phorcys and Ceto who were also married to one another. (The gods were very much into incest.) Phorcys and Ceto were sea gods who apparently had "a host of monstrous children." Monroeet theorized that the Monster and his sister are two of those children, especially because one — a half-snake, half-woman named Echidna — was notably killed on the order of Hera, queen of the gods.

Redditor sadb****esonly put forth an unusual idea: they believed the Monster's sister might actually be...the Monster. The memory that Penny visited was actually something that happened to the Monster, but because it was so fractured by the event, it believes it happened to its "sister" instead. It would be similar to what Harriet went through in the Mirror World. The Monster may be looking to become whole again after all.

The world of mythology is so vast and varied that the Monster's sister could really be anyone. Her identity may be a twist that fans won't ever see coming.