Purity of Heart

Purity of Heart is a defining personality trait some of the characters have in Relic of Light. It effectively means that the person in question has either a heart of pure good or a heart of pure evil, with no mixtures between the two, whatsoever. A character with this trait is said to be "pure of heart."

Overview
There are two ways of being pure of heart: to be either pure good or pure evil, without any mixtures between the two. It is extremely more difficult, if not impossible, to have a heart of pure good than it is to have one of pure evil, as dark traits (for example, pride and hatred) tend to surface even in relatively altruistic people. In addition to that, even being pure good doesn't necessarily guarantee immunity to evil.

Heart of Light
A person who has a Heart of Light has personality traits devoid of evil and malice. They have a desire to protect all that is good. However, it does not prevent them from developing a few dark desires (like lust and greed) along the way. There are a few ways to determine whether a person has a Heart of Light; It should be noted that while Lightsen has a Heart of Light, he has killed people, though most of those people were either villains or gods that are cruel to mortals in general. This is proof that one can commit potentially evil actions and still have a Heart of Light as long as their intentions for doing so are good or lack any malicious intent.
 * Heart of Light.png Divinity Scanner:  A godly power that allows a deity to see into a person's heart. This ability is often used to determine whether one is worthy of entering divine territory (such as the Heavenly Plain in Amaterasu's case) or not. The same applies to if someone wants to marry a god/goddess, though a mortal/god relationship is less common due to the Holy Laws forbidding said relationships.
 *  Nimbus:  Nimbus are magical clouds that allow only pure-hearted warriors to ride them, such is the case with the Monkey King and his (possible) current incarnation. Any person with mixed traits of good and evil will be unable to ride them.
 *  Brewing Tea:  It is a superstition made by the gods, but those with a Heart of Light are able to make tea so perfectly good that one drop could put a person to sleep. This is because the tea is free of any imperfections in its taste and drinkability.

It should also be noted that while those with a Heart of Light are not perverted in any way, they can still find women (or men if the person in question is female) attractive (both in appearance and personality) as certain characters are known to have had romantic relationships (such as Carlisle) and/or find women (again, or men if the person is female) attractive (Skyjet and Drilliam are prime examples). However, they care for the woman/man they truly are in love with and are faithful to the point of resisting any urge whatsoever to be unfaithful.

Dark Heart
A person who has a Dark Heart has personality traits devoid of good and virtue. They have a desire to destroy and defile all that is good. It is a lot easier to have a Dark Heart than it is to have a Heart of Light. A person with a Dark Heart can be tested out by the first two methods above, but there are tests that are unique to determining whether the person in question has such a pure heart;
 *  Brewing Tea:  It is a superstition made by the gods, but those with a Dark Heart are able to make tea so perfectly good that one drop could put a person to sleep. This is because the tea is free of any imperfections in its taste and drinkability.

Trivia

 * Interestingly, the Great Old Ones and Outer Gods having a Dark Heart makes sense given that they is the evil analog to the Arkeans who are well known for being pure good, with the sole exception of Sinai.
 * An individual with a Heart of Light can have some "poisonous" personality traits. For example, Carlisle's Zortonian love for battle can cause him to be inconsiderate at times.