Skills
Skills are the base aspect of the System, and to unlock a skill, you must want the skill and must already be capable in some form of whatever the Skill would provide. You could not, for example, gain the Fireball skill if you can't already cast a Fireball. Exactly what counts as sufficient capability and desire for a Skill to unlock is unknown, and seems to vary person by person. Some people find it really easy to unlock certain Skills, others might find it almost impossible. The one unifying theme of all unlocked Skills is that they are already something you are capable of doing, and leveling them just makes you better at what you can already do.
There is no hard-set skill cap, however, if you already have Skills with overlapping effects to the Skill you are trying to gain, it can be very difficult to actually unlock a new Skill instead of simply leveling your old one. For example, unlocking the Running skill can be very difficult when already possessing the Walking skill, as the skills are similar enough that practice running will usually level Walking instead. Note that this does not mean it's impossible, just tricky. In addition, it tends to be somewhat harder to unlock new Skills once you have a lot, but the point at which getting new Skills becomes impractially difficult seems to vary from person to person. If there is a hard limit to how many Skills a single person is able to have, it is not known.
Skills are not prescriptive. Just because you don't have the Walking skill does not mean you can't walk, not having the Swordplay skill does not mean you can't fight with a sword. However, it can be very difficult to keep up with someone who has the appropriate Skill, as any training they may do essentially counts twice; once to improve with practice (such as learning the proper forms of a sword style), and once with leveling (in the case of Swordplay, the skill improves proficiency with swords and damage dealt (essentially cutting power). As a result, most people don't think to try and do something they don't have a Skill for. Some do, though.
There is no way to remove a Skill once you have accepted it (though there are stories about anything from fey to devils to the elves which say they can steal Skills or remove them in some way, usually in the form of a cautionary tale about not being caught in the woods), though a declined Skill can be unlocked once again in the future.
Paths
Paths are, in a sense, the 'achievement' side of the System. Paths are unlocked for accomplishing something that the System deems notable. This can range from a Skill reaching some level to slaying a mighty foe or accomplishing something unconventional. Other ways to get a Path include being hired for a certain job/becoming an apprentice, learning some sort of information (or displaying knowledge about said information), finding a legendary treasure of some sort, being born a particular nationality or species, and more besides.
Paths have a cost associated with them, usually some value along Base 60. To complete a Path, you must a spend a number of 'Skill Points' equal to the Path's cost on the Path. You get one Skill Point each time you level a Skill. Spending Skill Points on a Path relating to an Attribute you have (see below) can improve the corresponding Attribute, but unlocking an Attribute does not retroactively give you the benefits of a Path which might have increased it (for example, any magic-related Paths completed before unlocking the Mana Attribute would not increase Mana, though later ones would).
If at some point you somehow manage to have no Paths left uncompleted, the System takes note of that, considers it an 'achievement,' and rewards you with a Path corresponding to your current Class.
Completing a Path has can have one of two confirmed effects. Either it upgrades/evolves a Skill you already have, or it unlocks an Attribute. In addition, completing one Path may remove another Path you have unlocked but not completed. Completing the Escapee Path will often remove the Slave Path from your options, completing the Traitor Path would remove the Loyalist Path from your options, completing the Killer Path removes the Pacifist Path, and so on.
It is possible to unlock a Path you have already completed. The Killer Path is the most obvious example of this, as you regain it every time you kill a sapient creature barring circumstances which would make you gain a more specific Path (such as Assassin, Executioner, Poisoner, or Murderer). Completing the Path again functions as any other Path, though naturally it does not give the exact same reward as the first time.
The tale has been illicitly lifted; should you spot it on Amazon, report the violation.
Attributes
Attributes are similar to Skills, in that they enhance certain aspects of the individual and can even overlap with and enhance certain Skills. However, they do not level through practice, only increasing by spending Skill Points on a related Path. Some of the more common Attributes, and their effects, include:
Constitution: Makes the body hardier and more capable of fighting off disease, poison, and makes recovering from an injury faster.
Charisma: Makes the person have more 'presence,' making them harder to ignore in some indefinable way. A wandering gaze will linger on them longer.
Dexterity: Improves coordination and proprioception throughout the body (though does not improve reaction time).
Health: Makes the body more resistant to injury and improves recovery.
Intelligence: Improves memory and pattern recognition capabilities.
Mana: Increases the amount of mana availible to the person before they are magically exhausted.
Magic Resistance: Reduces the effect of magical effects on the person. This can be lowered if the person knows how to, but by default affects both beneficial and harmful magics.
Perception: Improves spatial awareness and ability to handle additional sensory information.
Speed: Speeds up reaction time and improves how quickly one is capable of moving their limbs.
Stamina: Enhances the body's natural recovery abilities, allowing for physically draining tasks to be done for longer before the person is drained.
Strength: Enhances the body's natural muscle growth capabilities.
Willpower: Improved ability to focus and ignore mental compulsions, such as the effect of Charisma.
Skill Tiers
A 1st-tier Skill, or Basic Skill, is one that is unlocked individually. This is something that a person is already capable of doing in some form, and what most of Edwin's skills (at the moment) are. When a Path is completed, a Skill will usually be targeted, increasing its tier. This is in-universe referred to as either skill Evolution or a skill Upgrade, depending on who you ask.
The Skill chosen by the Path is usually one related to the Path in some way. For example, completing the Mage Path has a high likelihood of evolving the Mana Manipulation skill, whereas the Warrior Path is more likely to upgrade the Spear Proficiency skill. However, above all else, the Skill chosen is of the lowest tier skills that the individual has. If an Experienced Mage has ten magic-related skills at tier 2, and the Walking skill at tier 1, completing the Mage Path will always evolve the Walking skill, instead of upgrading Mana Manipulation, Mana Sense, Mana Shield, or any other tier 2 or higher Skill.
The resulting Skill after an evolution is based on the combination of the two Skills, filtered somewhat through whatever the individual's Class is. For example, a magic-related Path affecting a weapon Skill will often result in some kind of weapon enhancement skill, where mana is poured into the weapon and it improves its toughness, cutting power, and possibly even add even more exotic effects. Upgrading the Knives skill with the Killer Path would produce the Bloody Knife skill, which would make stab wounds with a knife bleed more, but upgrading it with the Cook Path would result in the Precision Cutting skill, which would make it easier to chop food in specific ways.
A higher-tier skill does have a higher impact per level (an indivudual with 30 levels in tier 2 Running would be somewhat faster than an individual with 30 levels in tier 1 Running), but it is a matter of in-universe debate how much of an impact it has and if it's worthwhile. Everyone agreed, however, that higher-tier Skills provide effects which often simply cannot be replicated as a 1st tier Skill. Teleportation is a possible upgrade to various movement-based Skills and is essentially impossible to unlock as a basic skill unless you are a master mage. The larger the Path (the more Skill Points it took to complete), the more drastically the original Skill is changed. A 12-point Path would change Walking to Running, for example, whereas a 120-point Path might change Firestarting directly into Fireball. Thus, those people with higher-tier Skills tend to have skills which provide very exotic, frequently abstract effects, whereas those with a lot of Skills at lower tiers are good at many basic actions.
When a skill is upgraded/evolved, the old effect of the Skill is not lost. However, it can no longer level.
Because I know I'll be asked: Most people in the Empire reach Tier 3-4 in all of their skills, with most young adults in Tier 2. Particularly skilled individuals tend to reach Tiers 5-6, and elites reach tier 7-8. It is rumored that Royals, individuals like Emperor Xares, demigods in their own right, might be as high as Tier 20.