The SOUL

Human SOUL
Human SOULs all have the innate ability to persist after death. It is called by Doctor Alphys as DETERMINATION.

Traits
As addition for the Human Souls, they have seven different traits, defined at birth and it's still unknown, what they are related to, because it was proven already, that an INTEGRITY SOUL is perfectly able to be from a murderer, even if it sounds contradictory.

Light Blue
Known as PATIENCE by the Ball Game, this trait is one of the six common human traits. Like the Red trait, there is determination that allows the SOUL persist after death, and not disappear like the Monsters SOULs do. There's no information of what would be the differences caused by a Light Blue Spell to change the SOUL color, but it's hinted to have an effect similar to the Blue Attacks but it's not confirmed.

Orange
Known as BRAVERY by the Ball Game, this trait is one of the six common human traits. Its determination allows the SOUL persist after death, not disappearing like the Monster SOULs do. There's no official information of what would be the differences caused by an Orange Spell to change the SOUL color. But it's accepted that it would turn the rules upside down. Meaning that White and Green Bullets would change roles, as well the Blue and Orance attacks. Reflecting the character's bravery to defy the battle system. Green Bullets will deal damage, White Bullets will heal, Orange attacks won't deal damage if the SOUL is still, and Blue attack will deal damage if the SOUL is not moving.

Blue
Known as INTEGRITY by the Ball Game, this trait is one of the six common human traits. Its determination, pretty much like the previous traits, allows the SOUL persist after death. If a Blue Spell changes a SOUL to Blue Color, a gravity effect is applied upon the SOUL, like a plataforming game.

Purple
Known as PERSEVERANCE by the Ball Game, this trait is one of the six common human traits. Its determination, pretty much like the previous traits, allows the SOUL persist after death. Rare are the cases, when it reaches the critical level of determination, that allows a timeline rewind in case of death. Even then, it's supressed if a Red SOUL is nearby. If used as Magical Spell, the SOUL is tied to purple strings that limit its movement on the battle box, like a car road.

Green
Known as KINDNESS by the Ball Game, this trait is one of the six common human traits. Its determination, pretty much like the previous traits, allows the SOUL persist after death. As spell, it immobilizes the SOUL, and in case of Undyne, a spear is given to block attacks.

Yellow
Known as JUSTICE by the Ball Game, this trait is one of the six common human traits. Its determination, pretty much like the previous traits, allows the SOUL persist after death. As spell, it turns the SOUL upside down, like the monster SOULs, and allows to fire yellow pellets that deal damage if fired directly towards a SOUL. This is the only trait of the seven, that has a counter-attack.

Red
The Red trait, also known by some fans as DETERMINATION Trait, is the most rare Trait known. Although trait does not influence on how the determination is expressed, it's hinted that the Red SOUL was the only one able to overcome the barriers of reality and manipulate the timeline at will, or near death. The Red SOUL can be taken as the supreme manifestation of Determinaion. The excess of determination in a Red SOUL, can even overcome the power of a God, being able to either SAVE, or ERASE. All depends on what they are determined to. A blessing, or curse.

Fear
There's no mystery in the fact that fear drives people to do awful things. Because it's easy to ignore something you don't understand, or don't want to. Although all this physical superiority regarding the monsters, humans still fear their power. This was the main cause for the war between humans and monsters, centuries ago. They couldn't stand a chance against the humans.

In General
The Snowdin "Librarby's" Orange Book details universal facts on Monster SOULs. Different from Human SOULs, Monsters SOULs does not have traits, or manifestation of any. They are also weaker, if compared to Human SOULs. It's said that hundreds, or even thousands Monsters SOULs are necessary to equal the power of a single Human SOUL.

Boss Monster SOULs
The only Monster SOULs that have been witnessed to exist without vanishing instantly upon a Monster's death, are Toriel's and Asgore's, due to being Boss Monsters. Only those types of Monsters have been stated to have SOULs that can persist after death, and even then, only for a few seconds.

Never Aging
It's said that Boss Monsters can't age after certain years (probably when the parents die), returning to age when they have a kid and their SOUL power flow into the child as it grows. This makes possible for Boss Monsters, to live eternally. This also explain why ASGORE was able to fight in the war between humans and monsters, centuries ago, and still be alive when the Seven Fallen Humans came, because Asriel died before they arrive, interrupting the power flow.

Special Cases
We may have characters that does not fit in the Human/Monster category, even because we may have anthros that aren't necessarily fitting in the "Monster" definition, either for not be composed mainly of magic instead of water, or because they're stronger than the monsters and humans already. Or we might have an in-universe special case.

Dragon SOUL
The Eastern Dragons are not originally from Undertale Multiverse. However, their physical abilities and determination, are naturally bigger than the humans. In the same format of monsters regarding humans, it's necessary a few dozens of regular human SOULs to be equal to the same power and determination of a Dragon SOUL. But this is not applied to the Red Human SOUL, being the only trait that have the necessary excess of determination, to get close, to the normal levels of power and Determination from a Dragon SOUL. Besides this disproportionate level of power, the dragon SOULs are similar to human SOULs in appearance, except for a Reddish Purple core, that reflect this new level of determination.

Glowing Aura
As a visual manifestation of their superiority, when a dragon enters in a battle, a glowing aura called PRESSURE keeps pulsing and affecting everyone involved in this battle. This Pressure suppress any power regarding the timeline, including SAVE, LOAD, RESET, and in case of the Red Dragon SOULs, the ERASE. It also reduces the opponeny's ATK a little bit.

Outside a battle, the aura is only visible if the SOUL is gathered from a Dragon, although this never happened. The power suppression still exist, but only for the Red SOULs. This also proved to be the only way to gather a human Red SOUL, without rewind the timeline, or have the "But it refused" event. But the former is only achievable with an incredible amount of determination from the Dragon involved, completely unnatural.

Monster with a Human SOUL
It's believed that a monster with a human SOUL, is the only being capable to overpower the strongest humans in history, and this would be one of the reasons for the war between the humans and monsters have occoured. Historically, Asriel is one of the monsters who absorbed a human SOUL, besides his fateful end