Ardor

Ardor (闘気 (アルダー), Arudā lit. Fighting Spirit) is a metaphysical concept that exists within the Sun Trilogy continuity. While not magical in origin or a supernatural energy per-se, is actually the user's determination and resolve made manifest; granting them capabilities beyond normal. It's a "power" conveyed by positive reinforcement—that is, by belief, either from the fighter's own self-confidence and unstable emotions, or the power of their loved ones cheering them on, or some other ambient effect.

It should be noted that proof of the Ardor power actually existing in reality is shaky at best—it has been referenced throughout history as a possible reason why many heroes were capable of standing to their feet and achieving victory despite all odds. The actual mechanism by which the user's determination or beliefs are transmitted and converted into energy are unclear, and are often inconsistent when attempted to receive an explanation.

Description
The Ardor power is a concept that exists within many beings—whether human or not. It has existed since the beginning of time, dwelling within all whom steel their resolve to fight to protect what they love. It is considered similar to magic in that for a vast majority, it powers them and allows them to trudge on, but yet it is not similar—it's seen as more akin to "life energy"- or rather, the force that powers all living beings made manifest through strong emotions. Essentially, it is described fairly vaguely as "a power brought into the physical realm specifically in response to a threat against something that the user cares about and thus their desire to protect".

When the Ardor power, well, empowers the one in possession of it, it provokes an enormous boost in capabilities all across the board; for example, it gives an ordinary human superhuman abilities like pulverizing steel with their bare hands, striking faster than the speed of sound, launching up to a hundred blows per second- the list goes on. But the more the user's fighting spirit is increased, the more extraordinary the power becomes and the harder it is for the fighter to control. At any rate, the Ardor power is shown to be heavily driven by emotions- and as such, an incredible willpower, or a desperate situation can influence its depth and effects. This means that when sufficiently roused, a fighter can surpass their limits a hundredfold, although without the right awareness, this usually takes the form of a momentary power increase.

Interestingly, despite its ambiguous-at-best existence, it is shown that the Ardor power is capable of affecting supernatural powers such as Magic and Curses—if a fighter's willpower is strong enough, most of the time, regardless of the power being used against them, if a fighter is particularly determined, then they have a small chance of flat-out ignoring the opponent's attack, even though normally, their body could not take it. This spreads to their own powers as well- for example, normally a Fire Magic spell is washed away by Water Magic, but if the fire user is filled with resolve, then their spell can potentially tear straight through the water.

It should be noted that it seems that Soul Metal tends to resonate with the Ardor power- it is said that only those with incredible willpower and determination to protect are capable of donning the Driver Belts which manifest Soul Armour. The Soul Armours seem to run on willpower rather than any supernatural energy- and as such, somebody with amazing amounts of resolve can run over the five minute time limit for the Soul Armours for a brief moment. In addition, it seems that the process of "Assimilation" is tied to willpower, and those with a "burning spirit" are capable of harnessing it for long periods of time. Indeed, when a Soul Armour Knight is supposedly influenced by the Ardor power, they often show undeniable physical alterations—e.g., colour changes, physical growth, new weapon attachments, or abilities they possess at no other time—which in turn can include powers that are all but miraculous, such as healing grievous bodily injury or suspending the laws of physics.

Trivia

 * As you can guess, the Ardor power is a snarky way of describing the tendency of protagonists to become stronger through seemingly nothing more than sheer force of will. It isn't a power itself.
 * Tsuruko Sejren in particular lives and breathes the Ardor power.