With everything that exists, there must be a balance. After all, the light can't shine without the darkness. Yin and Yang are always together. The world is not all black and white. It is an arrangement of colors. These balances were created before any AU's were.

So let's start with the least impacted balance. Good and Evil. Honestly, for some, it's just made-up nonsense. But it's very much a balance. With good, there must be evil and vise verse. In most multiverse's this role is taken by the Chara and Frisk equals. Some don't even realize this balance is needed, but it is. Most think that this balance is between the 'good' and 'bad' sanses but it's not.

The reason it is between the Frisks and Chara's is that there are a Chara and Frisk in almost every AU. The Frisks are the yang while the Chara's are the yin. This is due to most monsters blaming the Chara's even though they did nothing wrong.

The second least impacted balance is the one between Hope and Fear. It can also be called the balance between Positivity and Negativity. This role is taken by Dream and Nightmare. They are often brothers who used to live In Dreamtale before Nightmare snapped from the abuse by the villagers and ate the apples they once protected.

Dream is the Yang while Nightmare is the yin. Nightmare is the leader of his group and is now the king of what he is the yin of. His members throughout multiverses are Cross, Killer, Horror, and Dust. Sometimes Error, Blue, Red, Fresh, Geno and Razz are apart of the group.

The second most important balance is the one between Life and Death. It's not first for a reason that will be explained later on. The roles are taken by ReaperTale Toriel also known as Life and ReaperTale Sans that goes by Reaper or Death. Death was created alongside his brother when there was too much life.

Life is the Yang while Reaper is the yin. They did become friends and they are the most stable balance throughout the multiverse. Their father is Gaster who is the god of magic within Reapertale.

The last yet most important balance is the balance between Creation and Destruction. It is also the most unstable due to it being able to be completely stable one moment, but then a few hours later it is almost completely tipped. The roles are taken by Ink and Error.

Ink is the yang while Error is the yin. Error was formed by Fate -a deity who controls everyone's fate- to become the destroyer when the balance was tipped. The reason this is the most important and not the balance between Life and Death is due to it being the foundation of the entire multiverse. If there's too much creation, there's not enough room. If there's too much destruction, then there's barely and structure and the AU's will collapse on themselves.

All Yin sides of the balances know of them for they are there to balance out the yang from going overboard. All Yang minus Life does not know of these limits and balances however. Nor do they try to understand why the Yin even exists. Had they just asked once, then maybe there would not have been and unstable balances.