Castor's Dolls are many life-sized, porcelain dolls that Castor builds and controls with the use of his manipulation Zaiphon (only in the manga; in the anime he simply uses his Ghost strings to control them). Castor is able to keep a mental link with these dolls, and although he cannot see through their eyes, he knows where they are at all times, and can tell if one has been damaged or destroyed. They are lifelike in movement, are capable of making their own decisions, and react to their environment even when Castor is not present. They are difficult to break.
So far in the series, Castor has made dolls of himself, Frau, a nameless purple-haired sister, his mother and Razette. On one occasion, he also made a wider range of dolls for buyers to choose from at a Barsburg Church event. These newly made dolls' designs include a nun with long black hair, a dark blue, sleeveless dress and matching opera-length gloves; another nun with short black hair, a white undergarment and a pink dress with long, puffed sleeves, as well as a short-haired brunette peasant woman with a white head-covering, grey undershirt and a dark purple smock dress.
This hobby of Castor's first originated during his childhood, when he would build dolls in his mother's likeness because of the attention he craved, but he now uses them as surveillance and as helpers around the Church.
Castor displays an ability to merge his dolls with Kor. He does this by trapping the Kor inside the chest cavity of the doll and then sealing it with his Zaiphon to make sure the Kor is unable to escape. The Kor is able to function normally inside the doll, as well as it can when it possesses a real human, and is able to sprout wings.
There are two types of dolls that Castor builds. The first are the female, Sister dolls that are the more common type. They appear as females with shoulder-length purple hair and wear the typical nun's habit.
The second type of doll is the Frau doll that Castor uses during training, and these dolls resemble Frau in appearance (something done on purpose to encourage Teito to attack them), having his blonde, spiky hair. However, while his nun dolls have human-like faces, Castor's Frau dolls instead have a piece of paper stuck to their face with Frau's blank-eyed expression drawn on it. Strangely, they also display Frau's personality, as when Teito encountered his first Frau doll, it was striking odd poses and harassing the nuns as they walked by. This suggests that Castor is able to give his dolls personality.
They are very fast, and able to take a great amount of damage before they break.
- It is unknown if the appearance of the Sister dolls is based on an actual nun in the Church, or if their design simply comes from Castor's imagination, though considering that all the other doll types are based on actual people, the case is more likely to be the former.
- Kuroyuri once assumed that the Frau dolls Teito trained with were Teito's bodyguards.
- The Sister dolls show some fondness for Teito. One of them once held an unconscious Teito's head in her lap and stroked his hair, and another defended him from an attack by a possessed Mikage, of her own volition. The Sister dolls also seem to be familiar with Mikhail, shown when one of them smiled and handed Mikage's reincarnation to him.
- A Sister doll appears on the cover of kapitel 12, dressed as a maid.