Today, I'm making his Favorite to explain, EVERYTHING we know about the souls in UnderTale, and there are different souls, of course. And we are going to cover them all here and explain their characteristics. I'm doing this because there may be confusion in the fandom of UnderTale, especially about souls. I will do my best to explain every detail about the souls.

WARNING: IF YOU HAVE NOT PLAYED UNDERTALE, GO DO IT NOW! IT IS A GREAT GAME AND IS BEST EXPERIENCED BLIND! TRUST ME, IT'LL BE WORTH IT!

            ~The Human Soul~

The human soul, is the strongest soul in UnderTale. And it has to due with one main contributor. Determination. Determination, in the world of UnderTale, is a very overpowered thing to have. It's exclusive to the humans, and we'll cover why in a little bit. The human soul can have many characteristics, such as the Soul Of Justice, the Soul Of Happiness etc. The soul we use in the game is the Soul Of Determination, which belongs to our main protagonist, Frisk. The human soul is stronger than the Monster Soul, which is why they were able to win the war, with no lives lost. When a human dies, it doesn't shatter once they die, unlike their monster counterpart, due to their determination, mainly. If a human soul is absorbed by a monster, the monster will get more powerful, like we saw with Omega Flowey and Asriel. There is also the soul of hate, which takes an all black look and is powerful, Chara has this soul. There have also been a lot of Fan-Made souls, you could say. There are a handful of them in this Amino, which you should check out! Anyways, the human soul can also be extracted, from the host's body, which is probably why Asgore was able to keep the souls in separate containers, seen before the boss fight begins. The soul extractor itself can be seen in the true lab.

The soul (mainly Frisk's Soul)can also take different colors, in-game. The first time we saw this was in the fight with Papyrus, he turned your soul Blue, now making your souls gravitate to the ground. Making the game from a Bullet Hell, into a "Cat Mario" like experience. The next time we saw this was in the fight with Undyne. She turned your soul green. Which gave your soul a shield of some kind. The shield would be used to block the magical spears that she would fire at you. The next soul was pink, Muffet was the one who changed your soul's color. Your soul would hop onto lines that were in the box with your soul, the lines may have been webs, due to Muffet being a spider and all. He would be able to move on the pink webs, but nowhere else. The next soul was yellow, and Alphys helped you by installing an upgrade on your phone, if you press the yellow button on the phone, it would turn the soul yellow. The soul would now be able to fire, small yellow projectiles, or, since it's easy to remember, lasers! Those are all of the soul transformations in the game.

The barrier is what is keeping the monsters in the underground, the only way to shatter yo barrier is to acquire Seven Human Souls. But if the human can acquire a monster soul, they can pass through the barrier with it, but not shattering it, also note that the human soul cannot absorbed another human soul.

             ~The Monster Soul~

The monster soul is different from the human soul. It takes the form of an upside-down, white heart. It is weaker than the human soul, which is why they lost the war and got put into the Underground. The monster soul is made out of magic, while the human soul is made out of physical matter. The monster soul cannot be absorbed, but it can absorb a human soul, which would make them more powerful in the process. The monster's can't have determination, also, attempting to give monsters determination will result in their such a horrific failure. The monsters would start to melt or take a different, corrupted shape, then becoming amalgamates in the process, similar to what we see in the True Lab. Undyne also melted when she had too much determination.

The monster soul probably doesn't have 'traits' unlike their human counterparts, the souls have a white color, while the human souls can have many colors, like the seven souls of the children who fell into the underground.

                   ~Ghost Souls~

Something that is not often talked about is the souls that belong to that of Ghosts. Ghosts, of course have the ability to posses an object, in the game. When they're possessing it, they can leave the object whenever they want, this is when they're incorporeal. When they are incorporeal, they can't take damage from anything. The only thing that can hurt them, is magic, seen in the Mad Dummy fight. When they're incorporeal, they haven't fully "fused" with the object, meaning they can leave the object whenever they want. When the Mad Dummy fuses with the dummy in the genocide route, it becomes corporeal, one entity. This means that they can now take damage. Mettaton also did this in the genocide route, which is why you are able to kill him when he transformed into Mettaton Neo.

                   ~Conclusion~

This is all of the information about souls in the world of UnderTale! If I missed anything or made a mistake, please notify me if I did! Have a good day, fellow fans of UnderTale!