It's possible that some of this will change later in the future.
For the sake of simplicity, even though it's a medieval setting, the average life expectancy will still be about 80 for normal people. And magicians and ki warriors will live longer depending on their strength.
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KI (or warriors/assassin's aura)
Firstly, Ki can be manipulated in many ways. Being maybe a constant passive increases 1, a few, or many of the body's capabilities (boosting weapons is a branch of this method), an explosive finishing boosts increasing the user many times in many or a certain area, or solidification of the aura into armor or a weapon. Now onto the techniques.
Ki techniques allow ki, most commonly believed to be lifeforce, to be manipulated in a variety of ways beneficial to the user, if the technique is good that is. Techniques are usually gained through passed down teachings, blessings, meditation, self-study, experiments, or pure talent. Each with its own benefits and drawbacks.
Talent judges training speed, physique judges the full output of the user, and ki pool is basically the stamina bar for Ki, how much you can possibly use at once and for how long you can use ki.
Certain techniques were not built with the ability to cast magic in mind, and therefore do not allow it without severe repercussion. Which, depending on how intense the collision or how much Ki and Mana was, can range from a bare minimum of minor nerve damage to blowing a hole where the collision occurred. Which at times can lead to loss of limb, organ, feeling, and/or life.
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There used to be a Grading system but I ended up not Liking it so expect to know a little more about people's strength and power.
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Mana Link is the body's energy transferring link to the soul and mind. The lack of this link is what makes one incapable of magic, the link also varies in strength for some. The stronger the link, the purer the mana, the stronger the spell. Wider links mean more mana, meaning larger spells can be cast.
Since people capable of magic are about 1/500, not counting the strength of the mana, and Ki users are about 1/50, stronger link users are often absorbed into guilds, nobility, or other influential groups as a strong link is often capable of being inherited by their children.
The soul already has an effect on a person's temperament when they're born, this is the first link between mind and soul but it transfers information rather than energy. The Mana Link just allows for the soul to be capable of drawing upon the energy from the environment, a.k.a. natural mana. Well-trained mages will live longer than the average person (but unless they use spells and magic to prolong their life, a Ki user with an equal level of skill and training will live longer). Being charmed by magic or equipped with a slave collar is often regarded as a sickening, violating experience as it is a direct takeover of the mind and disregards the souls' temperament.
Mana levels can be naturally raised by attempting to train or further stabilize the mana link, expelling mana back through the mana link is a slow but effective process, though dangerous if done carelessly. Certain herbs, potions, and poisons will also work with varying effects. Mana can also be raised or strengthened through blessings.
Blessings are as they sound. Blessings given by gods. Can have a varying range of effects from strengthening ki or mana to giving control over an aspect of the god who gave the blessing.