Souls

Soul
Souls are the third most important things in the series for it to be used as to combine the color, size, and type to tell who is a human, witch, spirit, and many other different living species. It tells what the personality, power, and strength about the character. Another thing that is important about souls is that if the meister and weapon can compatible with each other's souls, they can be partners and meister will have the ability to wield their partner in weapon mode. However, this is rare and so far only the main characters (or maybe more) can partner up. When the perfectly partnered-up pair resonates (which is also called Soul Resonance), the meister's Soul Wavelength is sent to their partner's and then their partner sends back the wavelength to their meister. This goes on and on for a few seconds until the pair create a single and even greater Soul Wavelength that can unleash powerful attacks they can use.

Soul Wavelength
Just like in Soul Eater, each soul gives out a sort of energy wave called a Soul Wavelength. Each soul has its own Soul Wavelength and each one has a particular frequency. Some types of people, like Witches, all share a distinct Wavelength and can be easily indentified. To prevent this, they adopted the ability of Soul Protect.

Soul Wavelengths vary in strengths and frequencies, depending on how strong and big the soul is, and also on the health of the person concerned. Not much have been mentioned or shown about souls in the series but it was noted that Soul Wavelengths, like souls, seem to have some different types with certain properties, such as Hiba's Anti-Demon Healing Wavelength and Vincent 's Mind-Control Wavelength.

Soul Wavelength is the key element in Soul Resonance, and is also important in a Meister and Weapon partnership. A Meister and Weapon can only become sufficient partners if their Soul Wavelengths match with each other (see article Weapons ).

Types of Souls
There are a few types of souls which are:

Witch Souls: the soul of a Witch (or a general magic user). A Witch Soul is very powerful and has magical properties. Because of this, all Witches share a similar Wavelength. Witch souls are typically purple.

Human Souls: the soul of an ordinary human. Although individual souls vary greatly in this category (a Meister's soul is considerably more powerful than a normal human's soul), all humans ( Demon Weapons included) are under this category. Regular Human souls are a deep blue colour but there are some humans that have different colored souls, such as Jeffrey Ho, who has a green soul.

Evil Human Souls: the soul of an Evil Human. The soul of an Evil Human was once a normal human soul. However, by becoming evil and through consuming other innocent human souls, the soul of the human loses its humanity and turns into an Evil Human. If the Evil Human is left alone to eat more souls, it shall come under the risk of becoming a powerful Demon God like Death Mute. Evil Human souls are deep red but if they became a Demon God, their soul turns brown.

Demon God Souls: the soul of a Demon God. A Demon God's soul used to be an Evil Human's soul but the evil soul turned into a Demon God, in which their soul envolves into a much more powerful and bigger soul. If the Demon God consumes more souls, it will become deadly and almost undefeatable. Demon God souls are the color brown.

Consuming Souls
The only characters in the series that can eat souls are: Demon Weapons, Evil Humans and Demon Gods.

In the series, barely weapons eat the souls of Evil Humans or Demon Gods but starting from POV 4, Josie told the weapons in her group to eat the souls of Death Mute, Dark Heart Kills and Haley Hansen. Only Diego, Vincent and Diana are seen eating souls. Others absorb them by one of the parts in their weapons (ex: Shayan absorbs Evil Human souls through the pommel of his sword).

Later on the series, there are more Evil Humans consuming pure human souls and result of nearly becoming a Demon God. One of the most powerful Evil Humans were Vincent Ho, who needed three more human souls to become a Demon God. Demon Weapons consume Evil Human souls so that they can become more powerful and earn new abilities from the evil soul. However, they cannot eat human souls or they will become a Dark Weapon, which makes them go through the path of a Demon God. If the Dark Weapon consumes more souls, like Evil Humans, they will become extremely deadly Dark Weapons and can even take over their meister's mind and consume their soul.

Appearance
All souls, however are all different in appearance, and all of them share the same basic shape: a round ball about the size of a grapefruit (this may vary) with a flame-like tail, similar to the Hitodama depicted in ancient Japanese myth.

With enough power, an individual can cause their souls (along with their Soul Wavelength) to increase in size, up until they completely engulf their body itself. When resonating, their souls are also known to change shape.

Each person's souls differ in appearance, color, size, strength, the strength of the Soul Wavelength etc.

For example, Josie's witch soul is bigger than Yuki 's witch soul because Josie's soul is more powerful.

Trivia

 * The soul of a Dark Weapon is the same as an Evil Human's soul
 * Only Diego, Vincent and Diana are seen eating Evil Human souls