The Six Human SOULs are the remains of the six humans who previously fell underground and were killed by monsters. They were collected as per order of Asgore Dreemurr, to allow him to finally break the barrier, freeing monsters from their prison.
If Frisk chooses to spare Asgore during the neutral ending, Flowey will come out and deal the finishing blow. He will then proceed to absorb the human SOULs that Asgore collected, crashing the game. As the player reopens the game, Flowey will have corrupted the data and taken it over, molding the world into his image. When Frisk tries to SAVE the game, Flowey will come and destroy the game's save file, mocking the child about how he'll put them into a cycle of endless death, eventually engaging them in battle. At first, Flowey has complete dominance in the fight, with Frisk barely holding up and being unable to effectively damage him.
However, as each of the SOULs get into battle and attack, Frisk becomes able to ACT and calls for help. The SOULs will answer, beginning to heal Frisk instead of attacking. Each of the SOULs gets called out to one by one, and eventually begin to aid Frisk in their fight against the megalomaniacal flower. After the final SOUL gives its support, all of them will combine their powers to reduce Flowey's defense to 0 and heal Frisk as they finally are able to fight back. As Frisk brings Flowey to near death, he uses his LOAD power to simply overwrite the current timeline with an earlier version in which he was at full health. Flowey then proceeds to repeatedly kill Frisk, laughing maniacally and taunting them about how helpless they are. However, before he can deal the finishing blow, the six SOULs manage to rip themselves from his form, stripping him of his godly abilities and reverting him to his old self. From there, the player has the choice of either killing or sparing him. Regardless, the SOULs are stated to have gone missing after the ending, and Flowey will return when the timeline is reset.
During the true pacifist ending, Flowey will have taken the SOULs while Frisk and their friends were "having their little pow-wow" . The soulless flower binds the monsters in vines and starts attacking Frisk. Fortunately, Frisk's friends use their remaining strength to protect and heal the child, filling them with Determination as every other monster they have spared begins to show up and offer their support. Flowey takes advantage of this opportunity and sucks all of the monsters SOULs out, giving him power equivalent to seven human souls, allowing him to take his true form as Asriel Dreemurr. Thorough the entire battle, the Human SOULs are helpless to fight back. As the fight progress, Frisk manages to gets the SOULs of their friends back, eventually returning Asriel to sanity and allowing him to voluntarily stop fighting. Before returning all the SOULs he has taken, Asriel uses the power to finally break the barrier and set monsters free. In the end, the human SOULs have gone missing again, and the coffins of the six humans are shown opened, either hinting at a proper burial for them, or that they have been brought back to life.
If Frisk has killed everyone and let Chara takes control, Alphys will have called Asgore in an attempt to get him to absorb the SOULs and fight back. However, it seems Asgore failed to heed this warning, as Chara entered the castle and effortlessly murdered both Flowey and Asgore.
Powers and Stats
Tier: Unknown | Likely 2-B
Name: Human SOULs, Unknown individual name
Type: Source of life and culmination of one's being
Powers and Abilities: Determination, Soul Manipulation, Time Manipulation, Extremely potent power source, Regeneration
Attack Potency: Unknown (While Human SOULs have been described as extremely powerful, even matching all the monsters' SOULs, not all SOULs reach their fullest potential as Frisk did) | Likely Multiverse level (Allowed Flowey to achieve this level of power, also managing to strip him of said power after freeing themselves)
Material or Element: Patience, Bravery, Integrity, Perseverance, Kindness, Justice, Determination
Needed Prerequisite for Use: Being non-human. Although not needed for use, one need to be strong enough to completely control these SOULs. Someone with a single SOULs will have the control of their body split between them and the SOUL, while someone with 6 SOULs will possess much stronger control, but will still loses them if someone with enough determination calls out to them. 7 SOULs or higher seems to grants perfect control. These weaknesses seems to be non-factor if the user and the SOULs aims for a similar goal or otherwise)
Notable Attacks/Techniques or Alternate Forms: Light Blue SOUL (Patience), Orange SOUL (Bravery), Blue SOUL (Integrity), Purple SOUL (Perseverance), Green SOUL (Kindness), and Yellow SOUL (Justice)
Key: Individually | All 6 of them