The Seer / Atsuko Kadoya is an enigmatic cloaked figure who gave Kamen Rider Nile and Pecos their Henshin Belts. The show goes to great detail not to reveal who The Seer is until the end of episode 35 and offered red herrings to her possible identity.
Before the series:
Atsuko was born on an Earth where the Crisis Empire never invaded the Pra'meth homeworld and was the daughter of a Pureblood Pra'meth-human hybrid mother and Taro Kadoya, one of the great grandchildren of multiverse science researcher Sayo Kadoya. Her childhood was a happy one as her father protected their world as Kamen Rider Diend.
One day, the evil Pra'meths from the main Rider Universe were experimenting with Dimension Walls and the result caused their Earth to collide with an asteroid. Taro tried his best to save everyone when the Warship launched an assault, but after a long battle and several Kamen Ride and Attack Ride cards, the DiendDriver was destroyed. The Kadoya family used a dimension wall to escape but young Atsuko was angry at her father for running away, calling him a coward and berating him for not trying harder to protect their world. This caused her father to become depressed and her mother to scold her for being so hard on him considering all that happened.
When she became an adult years later, the doctors at a hospital told her that her father was lying on his death bed and wanted to see her. Taro told her that no matter what she thinks, he was glad she was safe and he was sorry he let everyone down. In his final moments, he said that he loved her and no matter what, Riders are a part of who she is and she should embrace that.
====During the series====
Taking her father's words to heart, she decided to travel to the main Kamen Rider universe after learning it was being decimated by the aliens who destroyed her home and assumed the guise of a ghostly figure to protect humanity. However, she soon realized that her power was not enough to stop the Pra'meths. Upon discovering two abandoned machines under a Pra'meth human slave colony that created strange orbs of tremendous power, she began building the CortalDriver and BramDriver belts to be used by two humans she foresaw as "The waters of Strength and Wisdom" to wield thier power in the hopes of freeing humankind and restoring the Earth to normal.
As The Seer, Atsuko resembles a large ghostly wraith with glowing blue eyes, a dark grey cloak and an eerie voice. Her normal appearance is equally as haunting, as she has albino white skin with asian facial features, frost blue eyes, a scar on her right hand and violet hair that glows in the moonlight.
Atsuko is one-third Pureblood Pra'meth thus she has enhanced strength and the ability to project her thoughts telepathically. Her human-Pra'meth hybrid DNA gave her other abilities, such as low-level telekinesis and sensing humans with unique attributes such as Gates or Plasmids. Like her great-great-granduncle Tsukasa, she uses dimension walls to teleport herself or others away from danger and has wide array of skills such as speaking the Pra'meth language and medical training.
- Sonozaki: In episode 4, she accessed the Gaia Library and held a book as she looked up the information, leading some viewers to believe that she was a descendant of Raito "Philip" Sonozaki from Kamen Rider Double. However this was proven false when it was revealed that anyone in Leone can access the Gaia Library via a port access terminal in the Leone Public Library that connects directly to the Earth's Gaia Memory.
- A Mage: In episode 7, she used an Invisible Wizard Ring and Excite Wizard Ring to fend off Kamen Rider Kahbur and used a rusted red Wizard Driver to cast the spells. However, a scan of her body by an Infantry-class Dorug revealed no Inner Phantom inside her but a concentration of mana, which baffled Kubick as to how that is even possible based on the data they gathered on Kamen Rider Wizard. After the fight, she had no detectable traces of mana in her body because she used it all.
- A Pra'meth: During an encounter with a Pra'meth fighter pilot that tired to abduct Seer in episode 10, she promptly beat him up and spoke in their native tongue, insulting him. It was revealed she had a scar on her right hand, knowledge of Pra'meth weapons and used a blaster against them with a great degree of skill. She also seems to have known about Rizzcot. This made the Riders suspicious of her, demanding that she tell them who she was and what her motives were, but she hesitantly declined stating that if she was a real Pra'meth they "would have been dead the minute they laid eyes on her".
- A Time-Traveller: One of the landmarks of outside of Leone is a wrecked train car that some of the citizens say fell from the sky when a bright light appeared. This white and black train car is a sleeping car with blaster burn marks on the sides and is equipped with a fully functional bath. After researching it, Pecos and Nile found out that the car may be from the legendary Train of Time, The Den-Liner. Seer states she uses this car as one of her "houses" to sleep in and does not want the Riders to spy on her. This lead them to believe that she was the survivor of a Pra'meth attack that might have befallen the Den-Liner and that she was from another time. They then speculated that the reason she hides her face is because she is one of their ancestors or because her direct interference with history would affect the timeline in a harmful way. It was later shown in a flashback in the movie that the Taros accidentally ejected the car during a battle with a rebuilt Gaoh Liner sometime during the 2030s. Unlike most red herrings in the show, this one is played more comedic given the nature of Den-O as a series.
- An Inhabitant of Mirror World/Fangire : The final red herrings appeared in the Non-canon Halloween Hyper Battle Special episode, the crew learn about the different monsters and villains that the Riders have faced over the years. They then see Seer walk through a mirror like wall and vanish, only to appear in front of them scaring them half to death. At first, they think she is a Mirror Monster who is toying with them until the time is right where she can eat them. But they then find out she has no reflection in mirrors and that she is in fact a Fangire in disguise from the World of Kiva that has come to eat humans before setting off to another world to repeat the process. The 3 Riders destroy her with their respective Rider Kicks, the real Seer then appears and congratulates the Riders for defeating the impostor and wishes the audience a safe and happy Halloween.