The Human SOULs are the SOULs that exists in every human. There is seven types of them, all in different color and different names. They are stronger than Monster SOULs, and if all seven are absorbed, the absorber would become a god.
In The Room of Undertale, they were kept in a sort of jars by Asgore, king of all monsters. When The Gang teleported there from Me, Flowey the Flower appeared and absorbed the Human SOULs, tranforming him into Omega Flowey. Omega Flowey would have won, except the Human SOULs decided to help the gang and put Omega Flowey's defense down to 0.
In The Lair of Dark Star, Flowey kept the Human SOULs, along with all the previous ones who have died's SOULs. When the gang came to challenge him, he absorbed all the SOULs he had and transformed into Asriel Dreemurr. Right after this, he quickly tranformed into his God of Hyperdeath form. Later on in the battle, he tranformed into his final form. When the gang managed to convince him to stop fighting, he tranformed back into his normal self and uses all the SOULs he had, including the Human SOULs, to break the barriers he previously set up when he was Flowey so all of the other villains would be helpless to stop his plan.
- They're similar to the Chaos Emeralds from Sonic.