In the world of Soul Eater, the human soul is ultimately the central focus. Realistically, the purpose of Shibusen, Shinigami and his Death Scythes revolves nearly entirely around the management of souls and thwarting those who would utilize their power for their own gain at the cost of others. To remain ignorant of how souls work and what exactly they are is to refuse a vital part of what it means to be a living, sentient being.
What is a soul?
Most Guests who are foreign to this world generally understand the "soul" to be a metaphysical or imaginary concept based in mysticism, faith or mere conjecture. That is not the case here. Souls are tangible and can be seen, and those with the necessary strength and ability possess the means to eventually become aware of their own soul and the power that it contains.
Everything that is gifted with life is also gifted with a soul. It contains its own type of energy that can, in certain types of beings, be utilized to directly influence either themselves or their physical surroundings. In a sense, soul energy is potential. Only a scarce number of people have the capability to eventually tap this potential, which is strictly dependent on the type of soul.
Types of soul?
The soul of every living creature is unique, however depending on what type of being they are its complexity and capacity will vary.
Human Souls are very plain and extremely similar between one and the next. While they do possess potential, they are neither strong nor complex enough for regular humans to harness in any way. Rare exceptions do exist, but otherwise humans are restricted to 'normal' capabilities.
They appear as little more than
blue spheres with tails.
Meister and Weapon Souls are the most complex. The ability to use the power of their own soul is what characterizes them as different from ordinary humans, granting them capabilities well above the norm. To those with Soul Perception, they appear very distinct with their own colour and detailed shape. They also release a Soul Wavelength, which appears as a blue spherical aura with markings derived from their soul.
EXAMPLE HERE.
Kishin and Kishin Egg Souls are typically similar to normal humans in that they are not overly distinct amongst themselves. They are vividly different though in that by devouring the souls of humans their own soul undergoes mutation as they lose their humanity and become more bestial and insane. Their soul will become strong enough to release a tangible Soul Wavelength, and more advanced Kishin Eggs (and Kishin) eventually develop the means to fully manipulate their own wavelength.
Kishin Egg souls appear as
cracked, red spheres. Here Kishin Asura is shown activating his Kishin Soul Wavelength. It is in fact much larger.
Witch Souls are similar to Meister and Weapon souls in that each is somewhat distinct and releases a Soul Wavelength. All Witches are the same though in that they release a shared type of unique Soul Wavelength that visually appears dark purple, but is characterized by markings related to their respective animal familiars. This wavelength is the product of the Sway of Magic, and is used for activating a Witch's magical abilities.
The Witch Medusa's Soul Wavelength is detailed with her vector snakes, whereas Arachne's has spider characteristics.
Other types of Souls do exist but are far less common or significant. A Shinigami's Soul is similar in many respects to a Meister or Weapon's, however it is significantly more powerful and they have access to a number of unique abilities. Immortal Souls belong to the mysterious Immortal Clan (who, as the name suggests, are immortal), and appear green. Animal Souls are borderline unnoticeable and too weak to possess any potential, however rare exceptions do exist.
...Soul Wavelength?
Souls with sufficient strength (Meister, Weapon, Kishin and Witch) release an aura that is called a Soul Wavelength. It is essentially the emanation of the soul's power and is what a being manipulates in order to activate their special abilities. It also serves as an indicator of their "power level" - the larger the Soul Wavelength is, the stronger, faster and perceptive they are, and otherwise governs their overall capabilities. The body, the mind and the soul are intrinsically linked.
For Meisters and Weapons, Soul Wavelength factors not only into the use of their own abilities but is a key element in Soul Resonance. Soul Resonance is the act of synchronizing the Soul Wavelengths of a Meister and Weapon in order to amplify their combined power. Therefore, Soul Wavelength Control is a vital skill to any being with a Soul Wavelength.
Wait a minute, I can already resonate and use Meister/Weapon abilities!
When a Meister and Weapon come into physical contact with another, what occurs is a very basic reaction based on their compatibility. The inexperienced and uninformed often make the mistake of assuming that this is all that resonance is: intuitive and empathic. In a similar fashion, Meisters and Weapons alike often find themselves tapping the inherent abilities of their souls instinctively rather than consciously, without fully understanding the powers that they wield. As if it somehow magically happens.
A Meister's abilities, a Weapon's abilities, and Soul Resonance are all a product of Soul Wavelength, and the difference between a new Technician and one who is experienced and properly trained is that the latter consciously controls their Soul Wavelength. It also marks the difference between a One-Star Meister and a Two-Star, or Three. Resonance and inherent abilities are stronger by far when one is actively concentrating on controlling their Soul Wavelength.
So how do I learn to control my Soul Wavelength?
In order to learn Soul Wavelength Control, one must first become aware of their own soul. Meditation is the main method of doing so - Weapons are routinely introduced to the concept when they are first taught how to transform, and this is also a common method in which Meisters discover their own abilities as well. The important part is to continue and take this process further.
The process of actually controlling one's own wavelength is something that comes with time and experience. The most basic way to start is to work on improving the strength of the soul by training. A new Technician's wavelength is weak and difficult to grasp. Beyond this, frequent use and practice with Meister Abilities and Weapon Transformation gradually exposes each to their own wavelengths, until such a time that they are able to begin to sense it.
The best and most reliable way to improve Soul Wavelength Control after that point, then, is through practicing Soul Resonance. Rather than simply picking up a Weapon and start fighting, however, the key is to focus on actually controlling their own wavelength, and in turn attempt to sense out their partner's. Actively synchronizing them is difficult and requires a great deal of concentration, which is why permanent Meister-Weapon partnerships exist and are maintained for years, and why the two are heavily reliant on each other when engaged in combat.
In truth, Soul Wavelength Control is something that a Meister or Weapon continues to learn over time, as it becomes more and more complex as their soul grows in strength, their wavelength grows in size and they develop new abilities.
How do I make my soul stronger?
A sound soul resides within a sound body and sound mind. This is not only the ethos of Shibusen, but the path to becoming more powerful. The soul naturally grows stronger over time when housed by a healthy body and a sane, disciplined mind. The three are inter-connected, and thus it is also important to actively train all three aspects.
In addition to the methods mentioned prior to improve the soul and control of its wavelength, one needs to also consistently train their body physically, as well as study and grow more learned and intelligent. Feelings are also a major aspect of the mind, and dealing with the likes of personality flaws and emotional issues contribute to stabilizing the soul.
When the three are balanced in tandem, the remaining element is merely time. Cultivating the soul is a long-term process which can take years.