Characters' strengths are classified by a class: E, D, C B, A , and S. E is the lowest and S is the highest. There are divisions to the classes; they can be separated as low, medium, and high or simply B-, B, and B+. There is no difference in classes when it comes to the different types of Spirit Energy. Seikouki, Youki and Reiki fall into this class system.
The weakest class of all. Demons of this strength aren't strong or smart but they are incredibly violent. However, their strength is just above that of the average human. Most humans and Spirit World residents fall under this category.
The second to lowest class. Like E-class demons, they usually aren't very intelligent (with few exceptions) or exceptionally powerful; but their strength and endurance is super-human compared to an average human. However, compared to most demons they are very weak. According to Koenma, Hiei was a high D-class when he first fought Yusuke. The strongest normal people with spiritual awareness in the world are all D-class.
This is about average strength for most demons. They are stronger than E and D classes, but their power pales in comparison to the stronger classes. These demons are very common and are relatively abundant in both the Human and Demon worlds.
This class is where the strength level skyrockets and is the beginning of the upper class demon strength, where the difference between the previous classes is absolutely enormous. B classes are far more powerful than C classes, but they still can't compete with the higher classes. This is in most cases the strongest class of demon that can be found in Human World, as any higher class can't pass through the Keikai barrier net between the human and demon worlds. The only time a higher class demon will be found in the Human World is if the Barrier is down or a higher class of demon wears an organic device that downgrades their power, as Hokushin did when he first met Yusuke. According to Koenma, Younger Toguro was an upper B-Class demon when he fought Yusuke at the Dark Tournament.
The second to highest class, A-class strength level are almost exclusive to Demon World while the Keikai barrier is up; the only exceptions are if a human attains this strength, a demon achieves this level in Human World, or if the barrier is destroyed and demons with this power can traverse to the Human World. Prior to the Keikai barrier's removal, A-class demons could not move between the worlds without aid. While exceptionally powerful and rare in the Human World, they are relatively common in the Demon World. However, compared to the power of an S-Class demon they are very weak; so weak in fact that they are used as mere foot soldiers of the Three Kings. The legendary spirit fox, Yoko, had been an A-Class demon and the feared fire demon, Hiei who reached A-Class at the age of five, Their power easily surpassed that of Younger Toguro's prior to their situations where they both temporarily lost their immense power.
S classes are the highest class (the pinnacle of power). They are so strong that even a "low" S class' strength can completely devastate a large area. In the Demon World, they are rare; with only about 20 ever known by name. It is extremely rare for even an A-class to achieve this level, however it has been observed that a strong A-class could achieve upper S-class in less than two years. A single Lower S-class is strong enough to defeat a horde of five-hundred upper A classes by themselves with relatively little effort. Sensui's full power was said to be at this level, as was Yusuke's when he came back to life as a demon. Kurama and Hiei also reach the S Class during the Three Kings saga.
- S and E classes are very rare, E class are so weak that they simply don't last long and technically don't even show up on the Spirit World's chart, S class on the other hand, are the opposite. Their strength is so rare that even Spirit Worlds' elite soldiers don't know about them.
- In both the anime and manga, C Class has the biggest roster of characters in the series.
- Although there are only three types of energy that a living being can utilize in the series: spirit (reiki), demon (yoki), and sacred (seikoki), only two are shown able to reach the S class level, demon and sacred. Demon energy (yoki) does not seemingly undergo any changes, remaining relatively constant throughout a demon's lifespan, though it will still grow in both strength and amount possessed. Spirit energy (reiki), however, reaches a point where it ceases to rise at certain point or time in a human's life, such as in Genkai's case, when she entered old age. However, with the right amount and type of training, a human's spirit energy will reach a point where it undergoes a metamorphosis, and fuses with a person's body. In this instance, spirit energy will change and become sacred energy (seikoki), enabling the wielder to reach S class levels.
- The A class is the second most rarely seen class behind E and S, as most B class demons do not make it to A class, and some of the time when they do they rapidly progress to S. With some individuals only takes a few months to years to reach lower S class level class levels once in A class, with proper training from strong individuals.
- Most higher level demons consider the spirit world class system to be insulting as the S class category merely refers to demons they do not have the power to fight or stop and demons they consider S class range from simple foot soldiers to demons that are considered to be like gods. Obviously the God powerful upper S class demons do not like being put in the same category as their underlings.