- "They are "Ultimate" mechanical boys. Five senses reach into five dimensions and four limbs extend into four dimensions. That adds up to the power of nine dimensions."
- ―Roger Dunstan about Ultimo and Vice[src]
Throughout the series, only a total of 16 dôji have been shown, though Dunstan has supposedly created 100.
The term Karakuri Dôji literally means "mechanical boy" in Japanese. The mechanical boys themselves are often referred to as "dôji".
While dôji are very diversified in their appearances, they all share certain traits. One trait would be the mechanical gauntlets that all dôji wear, which typically extend to their elbows and wear transparent extensions on their faces which are called "visors". Their clothes are usually of the highest quality, and typically consist of a hakama, a sash, and a piece of cloth from their sash that bears a symbol iconic to that dôji. They have also shown to have flawless porcelain skin and stoic expressions; another trait that makes them look like dolls at first glance.
They have the inside structure of a robot, but look very human-like on the outside, and have unique eyes and pupils. When they are not moving, many characters have mistaken them for dolls at first glance, but when moving, they are mistaken for children, since majority of the dôji are short and young-looking; to the point of having an androgynous appearance. While most are short and slim, others can be larger in height and have a more bulky figure, such as Goge and Orgullo. They can also look like regular humans by hiding their visors, gauntlets, and wearing human clothing.
In each dôji, they have their "spirit sphere", which are the souls the keep the them alive. It's also their source of power and what links them to their masters, should it ever breaks, the dôji would "die". The souls within the dôji plays a vital part in the One Hundred Machine Funeral, in which each opposing sides must change the states of their opponent's soul. The spirit sphere share a balance that similar to Chinese's yin and yang - colors of the soul is white for good and black for evil. The spirit sphere is located within a radius of about three centimeters behind the lower part of the sternum of the Xiphoid Process.
As sentient and untainted beings, each and every dôji has their own unique personality, much like humans. However, a dôji's personality often reflects their alignment and the trait they represent (i.e. Avaro represents greed, and shows signs of being a greedy individual).
They can either pure good or pure evil, but are incomplete and designed to learn about human emotions and form personalities from their respective masters.
As robots, dôji are far stronger and more durable than normal humans, and are capable of flight. They are also generally equipped with enhanced perception such as sight; often a dôji can hone in on their target close within their proximity through a point of view featuring hexagons. They can also look into the memories of people, as demonstrated by Ultimo on his master, to determine if that person is worthy to become the dôji's master.
A dôji's power mostly comes from their master's love, and can heal faster if given some, such as when a dirty Ultimo hugged his master, Yamato Agari, he suddenly became clean and shiny. Although, this work differently on evil dôji, they draw their power for their masters' hate or desire. In terms of power between The Good Dôji Club and The Evil Dôji Branch, everything must be even. Ultimo and Vice have equal stats in power, but all the Six Perfections are slightly stronger than the Seven Deadly Sins since they outnumber the good dôji by one.
- Karakuri Henge - Literally means "mechanical change", Dôji typically use their gauntlets in combat and capable of transforming them into weaponry, which are animal-themed and are named as such.
- Noh (能 Nō, "skill" or "talent") - Each Dôji also wields a unique power that they are capable of using, which can only be fully obtained and excess to its' to the highest point after the dôji does the Pledge Ritual with their master. According to Jealous, in a battle between Noh, the highest energy output wins, but precise timing at using their Noh can also contributes a battle's victory. Their Noh comes from technology from The 30th Century made by Dunstan, and some Noh powers can be cancelled out if one possess an Anti-Noh device.
A unique type of Karakuri Henge transformation that seems exclusive to Ultimo and Vice. To unlock this power, Ultimo and Vice must gather a large amount good or evil energy respectively from their masters or any other sources, converting this energy to allow them "evolve" and become a "second".
Once achieve, Ultimo and Vice gains a new dôji form with a new set of clothes, which they can freely shift to and from their usual forms. This second form allow both Ultimo and Vice become much stronger in many ways, including new abilities and an upgrade version of their ICONs with their masters.
Dôji are only able to reach their full power when they perform the Pledge Ritual with a master and become extremely large robots called ICONs. This is a dôji's most powerful Karakuri Henge, in which dôji physically join with their masters. In their ICON forms, good dôji are titled "God" (e.g. God Sophia) and evil dôji are titled "Demon" (e.g. Demon Spider), then is follow by their name or animal theme. Vice is the only dôji that is able to perform an ICON without a master, due to the designs made by Dunstan for him to represent the "Ultimate Evil" with no restrictions, but his ICON is arguably less powerful without a master.
ICON take many forms depending on the dôji's Noh Power and the master's characteristics, but it require immense amounts of energy, which allude to ICON's large size. Upon transforming, the ICON grants the master many use techniques and maximize their physicals abilities. The ICON is controlled by the master's movements, meaning the dôji cannot object to their masters commands even if they want to in these forms. Since their nerves are link to the ICON, both parties feel pain and vice verse. When the master is in ICON mode, they are connected to the ICON by mechanical tentacles cords that goes into their bodies, which are very painful, and are place in the head of the robot, which serves as the cockpit.
Normally when transforming into ICON, all of the master's clothes are shredded, but once a master and dôji's wills reach a unification of 95% or higher, they will then go into the second step of ICON and gain a leather ICON suit. It's a strict condition, but this allows remote operation within a radius of 50 meters, no longer having to wear the painful cords in their bodies, allowing the the master to be able to walk outside and control their dôji's ICON if they wish to.
Once a Dôji Master reaches this stage, the dôji's ICON form changes to match the preferences and abilities of the master and the master's given name is included in the title (e.g. Makoto: God Regla). It also has an output increases of power to an average of 120% than a normal ICON, improving speed and dynamic vision to the ICON. The exact style of the suit varies from master to master, but all ICON suits has the three tomoe on their clothes, the orb with the master's Karakuri Crest symbol circling around them, and light spikes protruding from their bodies that acts like a wireless LAN.
The Dôji system requires only one of each model exist to for all space-times, meaning if dôji travel to the past or future they cannot leave the time dimension as the dôji of that timeline already exist. Also, the amount of strength the dôji has depends on the bond between the dôji and his master. If there is any mistrust, doubt, or faltering from the master because of his or her dôji, the dôji will have a limited source of power.
The only way a dôji can truly "die" is if their spirit sphere is crushed - as it is their source of power and their bond with their masters. Also, if their masters are seriously injured or dies, they will become powerless until the master is heal or they find a new one. Every time a dôji uses their abilities, they expend Aizo (愛憎, "love hate") Energy. If the the dôji spends too much power, they will have to rest until they get their Aizo Energy back or receive it from their masters.
As a limited put on dôji by Roger Dunstan, they must have masters to learn about good and evil, and use their full power. A dôji's power comes for their master's love/hate or desire. Depending how strong love/hate or desire is and the closer in range they are to them, the dôji will also become stronger and could use their powers to its' most extreme level. When a dôji chooses a master, that person must possess a certain trait related to their virtues or sins to make them suitable, so they can learn what is good or evil and have an understanding on their virtues/sins.
Although a dôji must always obey the masters' orders and protect them, the exact nature between the two depend on their relationship, which can be either strained or close. Tomomitsu Iruma pointed out that a dôji's service to their masters only last a short time since the reason they were created was to see which is stronger, good or evil. As all the masters are human, they die more easily than their dôji, so Dunstan had planned many "back-up" masters if one dies. According to Musashi Murayama, Dunstan carefully picked out the masters for the dôji and hinted that each master, regardless of affliction, would have the potential turn good or bad to fit the terms of the One Hundred Machine Funeral.
- Sometimes, the dôji refer to each other and Dunstan by a familial title; Jealous once called Ultimo "brother" and in the USA edition, the dôji have called Dunstan their "father". Dunstan refers to himself as the dôjis' "father" as his their creator.
- Although there were some gender confusions due to their effeminate appearances, all of the doji revealed in the series are male.
- Despite being robots, the Karakuri Dôji have been shown to eat and drink like humans.
- Fusataro Fussa mention since that the Dôji are robots, they have perfect singing voices and music skills.