So the magic system in Errant works like this (oh boy, you asked for it):
Magic is a type of energy. This energy, like most forms of energy can be converted into other forms of energy, ie. kinetic, light, sound, heat etc.
Like electromagnetism, magic comes in different wavelengths and the stuff humans can use is just a narrow spectrum. ie:
Fey beings have much denser magic than humans, with a short wavelength, and so can easily vanish from human view, and they can do what scientists might refer to as "freaky shit", like warping your soul in ways that could affect your perception or personality, or even cause you to mutate physically. You shouldn't mess with them.
Demons are comprised of mostly visible and long-wavelength magic, so they mostly do stuff like cobbling themselves together a rudimentary body from dust, dirt etc. then smashing things. If there isn't much ambient magic, demons can't form a body and aren't much of a threat, so really you don't need to expect to see them in town unless some asshole takes the shiny magic sword that kept a hole into the demon dimension closed.......... Oh, wellllll crap.
The longer wavelengths don't travel so far, but have a powerful effect on physical bodies and objects. So if you happen to have a lot of magical energy clumped at the red end of the spectrum like a certain Ms Lune, you would be able to augment your own body to be incredibly strong and durable... but be really, really bad at projecting magic outside your body, so you'd be bad at healing, sensing magic in your environment or using cool sword draws that shoot blasts of energy, because the low wavelengths just don't travel well outside you. Meanwhile Sarin Aoki has a mostly blue-leaning profile; she is brilliant at projectile sword draws and highly sensitive to spirits, but can't augment her body well, so has to be careful to keep her distance.
Being a good healer means having wavelengths that affect flesh, but can also be projected to affect other people, so healers tend to be in the middle, in that green area of the human part of the spectrum, like Subo Talwar!
The amount of magic a person has is measured in Tans (Ta), named for the Nobel Arcane Science prize winning Doctor Tan. Magic is generally produced by a person's soul and integrated through the nervous system. A completely average person produces about 50Ta , but exposure to high levels of ambient magic, particularly when young, tend to acclimate by producing more of their own wavelength to prevent foreign wavelengths from negatively affecting their soul and ego (well either that or they turn into demons.........). Most human bodies can only withstand about 200Ta before they start to accumulate tissue damage, so it's a matter of some discussion in the Arcane Science community that tests seem to suggest Elrekia Lune has a magic count well over 200.
The magic that suffuses your body will cause your hair to appear an unnatural colour. The brighter the colour, the more magic the person has. The effect does not change the pigmentation of the hair, it's based entirely on magic interacting with light, so if the person wears a limiter, blocking their body's production and absorption of magic you'll see their hair's natural pigmentation. Sarin's hair is actually black, it's just suffused with magic that's reflecting blue light.
Magic in Britannia is often used for sword draws. This style of fighting is also popular in some other countries where swordfighting is a proud tradition and involves keeping a sword close to the body to absorb magic in a specially-treated scabbard, then unleashing it in a powerful burst. Most cultures have some kind of formalised system for shaping the draws into specific effects, the British way is runes, but some might use mantras, words, hand seals etc.
