A type of majin possessing high magic power and the ability to cast a variety of spells, although they have a very young, childish appearance. Originally, they were human women, but they were lured into receiving a baphomet's power and becoming monsters by various temptations such as eternal youth and high magic power, etc. As might be expected given that they were originally human, most of them blend in and live among humans, and their temperament is also relatively close to human, but they've been completely indoctrinated in the ideology of Sabbath, a pagan cult led by baphomets. Under that ideology, they perform various activities such as experimenting with bewitching spells, propagating Sabbath, and capturing human men.
In accordance with the doctrine of Sabbath, which values “the depravity and charm of little girls”, they all uniformly have the appearance of little girls, and they never age any further than that. Likewise, following that doctrine, since they always continue to be little girls, most of them speak and behave the way they look, like children. They are all baptized in devilish pleasure by a baphomet, and in accordance with the teaching of Sabbath that says “devote thyself to pleasure like a monster”, what they love more than anything is to use their childish bodies to go wild having sex with men bigger than them. Plus, by obtaining the spirit energy of human men, they also replenish and empower their magic. When they spot a human man that strikes their fancy, they'll actively seek intercourse.
According to the doctrine of Sabbath, they'll thoroughly instill the body of the man they like with the wonderfulness of the pleasure provided by their childish body and the charm of little girls. Afterwards, they'll form a master/ servant pact and have him become their familiar and “onii-chan". The head of a witch little sister with an “onii-chan" will become filled with thoughts of onii-chan. She'll end up spending every day having sex with onii-chan and indulging in pleasure.
Periodically, they gather before a baphomet and hold an assembly called “Black Mass.” There, they unveil new familiars; familiars and witch candidates who haven’t completely been indoctrinated by Sabbath, and human men and women who were gathered from villages through proselytization are thoroughly trained and educated in the ideology of “Sabbath", and then, it is said a feast of pleasure will unfold where witches and familiars who love each other will have sex with each other as they desire in order to reaffirm “the charm and depravity of little girls."
As the name “witch” suggests, they stand out even among monsters as having high magic power. Along with baphomets, they conduct research for the sake of developing new spells and magical items. Naturally, they are also proficient in attack magic, etc. but most of their demonic energy is used on charm spells and for making life with onii-chan even more pleasurable.
TL Note: ”Witch” in Japanese, 魔女 “majo”, literally contains the character 魔 “ma” which can mean magic, hence the "as the name 'witch' suggests" line.