In Soul Eater, witches are powerful beings and the natural enemies of Shibusen, as almost all of them have a destructive nature. Most witches featured in the series are based on a distinct animal and have some of their traits in both their personalities, fighting styles and appearances. Witches are also distinguished by their uniquely-purple soul.
Sway of Magic
This is a term used to refer to the naturally destructive instinct that occurs in both a witch and her magic. Almost all witches are influenced by this nature but there are a few exceptions. At a young age a witch's magic has not yet fully developed and so the sway of magic has yet to take hold, but as they grow older and their powers develop this destructive instinct strengthens. Witches who also use nondestructive magic are exempt from this influence but those who fall under this category are extremely rare, with Kim Diehl being the only one present within the series so far.
To make use of certain spells, most commonly when they are initially demonstrated within the series, numerous witches (and other beings capable of using magic) perform a specific mantra or chant preceding the statement of the desired ability. Each witch has her own unique phrasing that often pertains exclusively to their respective animal theme; for example Medusa Gorgon's mantra comprises entirely of words relating to her theme of snakes, being "Nake, Snake, Cobra, Cobabara".
However; despite the various instances that follow this magic casting system, it is apparently not a requirement for all witches as both Arachne and Mizune are notable for the absence of any such statement, even though both have used magic multiple times throughout the series.
Each witch has their own abilities that are as unique as their animal theme. However, all witches share a few common abilities.
- Soul Protect: As the witches' soul wavelength can be easily distinguished by trained meisters, they developed an incantation named "soul protect" which conceals their presence, but renders them unable to use their powers unless the incantation is lifted.
- Flight: Most, if not all, witches have the ability to fly. This done by simply floating in the air, or flying on a broomstick.  
- Spiritual Flexibility: There have been instances in which witches have a high understanding of soul wavelengths -- enough so that they can break their soul into small fragments and reform in due time, as emulated by both Medusa and Arachne.
- Meister Capabilities: Although it is unlikely for a witch to partner with a weapon due to strife between them and Shibusen, it is feasible for a witch to use a weapon. Kim Diehl, for example, is a lamp meister.
- Longevity: Witches, at least the powerful ones, appear to have incredibly long life spans. Medusa is known to have been alive at least 800 years before the start of the series. Even after eight centuries, Medusa still has the appearance of a young woman. Both Risa and Arisa confirmed that witches have a longer lifespan and so have more youth.
- Portal Manifestation: In chapter 98 it is explained that witches can easily conjure portals to the witches's realm. Apparently there is a rule that the youngest witch must open the portal. Kim opens a portal by lifting her skirt and wiggling her butt in the form of hiragana. Kim states that the portal has to be made through a witches body leaving a question as to the phrases meaning.
Medusa is one such witch who also acts as the main antagonist. Another witch who gains importance in the story is Arachne, Medusa's older sister and leader of the organization Arachnophobia, which poses itself as a serious threat to Shibusen. Not all witches are antagonists, however, such as Kim Diehl, a weapon meister from Shibusen specializing in regeneration magic, and Angela, a child witch under care of Mifune, a skilled swordsman who is determined to protect her at any cost. The witch queen Mabaa is known as the strongest witch, and being nearly as strong as Shinigami.
Witch Mass is the name used to refer to the gathering of a large number of witches, conducted under the authority of the current witch queen, Mabaa. Little has been revealed about the exact nature and purpose of these events but these meetings are considered as sacred to most members and it is seen to be a large disgrace to have an outsider (non-witch) accompany a member to one.
Each mass ends with the phrase "Joma Joma Dabarasa" (backwards for "Majo Majo Sarabada", which is "Witch Witch Farewell" in English), firstly said by Mabaa and then replied to by the other attending witches.
Relationship with Weapons
|“||In the old days, Witches were far more powerful and caused much more harm with their spells than today, and they were very clever. In order to oppose them we desperately needed a new kind of power. What Eibon invented to solve the problem was said to be modeled after Excalibur himself: the Demon Weapon. Of course, Eibon denied it, but weapons are power! Everybody knows that. To merge an actual soul with a weapon would release incredible power. Therefore, it was necessary to make a weapon into something that could absorb souls. Excalibur can't do that, of course. There was no way of turning a human into a weapon, so we created a human and weapon hybrid. And since the Witches have the power to shape-shift, we thought a Witch's soul could be used as a binding agent to create the connection between human and weapon. But such an experiment would be monstrously unjust, so Eibon and I sealed up all the information about that theory. [And it was the Witch Arachne who actually performed the experiment.] Arachne didn't just create a single weapon, she mass-produced them. And now there are many Demon Weapon children born with the genes that allow them to change into weapons. We were the ones to find those children, gather them, and train them. Without a Witch's soul, those children were incomplete. That's why a Witch's soul is needed to become a Death Scythe. Using the Demon Weapons that were born from a Witch's soul, we were able to hunt down the Witches and collect their souls.||”|
—Lord Death, Chapter 99
After a Demon Weapon has consumed 99 corrupt human souls, they can consume a Witch soul to become a Death Scythe. The process is very specific; if a soul other than a Witch Soul is eaten last, such as a Cat's soul, all the previous souls will become worthless and the quest must be restarted. Also, the Witch soul must be consumed last, as seen when Soul had to wait to consume Arachne's soul, even though he had obtained it before consuming ninety-nine other souls.
- Both Angela and Kim Deihl share a comparison where they have no actual biological parents shown or mentioned in the series (not counting Angela's father-figure, Mifune). This comparison draws two different conclusions: either both parents were killed when Kim and/or Angela was at a young age for being witches, or that witches are naturally separate from their parents at a young age, explaining Kim's easy decision to deviate from the witch world onto Shibusen and Medusa's contempt toward her child Crona. The latter may be a reference to the Miyazaki film Kiki's Delivery Service, where all young witches coming of age must live on their own for a certain period of time.