Eight Humans

The fallen Eight Humans are the only humans from the surface that have fell through Mount Ebott into the Underground. They include the First Human, who is commonly refered to as "Chara," the six human souls that Asgore has within his grasp, and the playable character.

History
Prior to the events of the game, seven humans had fallen into the Underground. The first of which was befriended by Asriel after being adopted by Toriel and Asgore. However, they died relatively soon thereafter. Asriel absorbed the human's soul, allowing him to cross the barrier; he carried the human's body to fulfill their last request of seeing some golden flowers, which he found at a human village on the surface. The humans mortally wounded Asriel, who returned to the Underground with the human's body before succumbing to his injuries.

The shock of this tragedy caused Asgore to declare that all humans that fell into the Underground were to be killed and were to have their souls collected for the purpose of permanently destroying the barrier, and waging war on humanity.

Sometime between then and the start of the game, six additional humans fell into the Underground, all of which were killed, and had their souls stored for use by Asgore. Their bodies were kept in coffins near Asgore's throne room.

Main Story
Before initiating combat in Waterfall, Undyne tells the player that six humans have had their souls taken from them, and that the player's soul will be the seventh and final one required to break the Barrier.

Later in New Home, Asgore reveals the six souls he possesses right before attacking the player. After he is defeated, Flowey absorbs the souls to become a powerful creature.

Neutral Route
During the final boss sequence after Asgore is defeated, the six souls and their items are used to attack the player one at a time. If the player calls for help through the ACT option offered in battle ('You called for help...'), the soul will assist the player by using green attacks which heal them. After receiving help from all six souls, they reduce Flowey's defense to 0 and regularly send green attacks to the player. After he is defeated, Flowey loads the game to a SAVE file when he was at full health, but the souls revolt again, removing his abilities and leaving him helpless to the player.

Sans tells the player after the credits that the souls have disappeared, precluding the plan of breaking the barrier through their use.

Genocide Route
In the genocide route, Alphys urges Asgore to absorb the six souls. In the ending, Chara appears in person (having fully possessed Frisk by now) to speak with the player.

True Pacifist Route
Toriel points out that Asgore could have absorbed a single soul to cross the barrier and find six more humans to collect the souls of if he really wanted to break the barrier.

Flowey steals the six souls and absorbs the monsters' souls to regain his form as Asriel. He then fights the eighth human while mistaking them for the first human. The eighth human saves Asriel, who accepts that the first human is gone and learns that the eighth human's name is Frisk. Asriel releases the souls after destroying the barrier, allowing Frisk and the monsters to leave the Underground.

The seven coffins near Asgore's throne room are opened, either to prepare the humans' bodies for burial or indicating that the humans had been restored to life with the return of their souls. The first human's coffin is empty, as Toriel buried their body in the Ruins when she moved there.

List of Human Souls
The following is a list of human soul colors of the Eight Humans and details associated with these colors.

Red Soul

 * Associated Items: Stick and Bandage at beginning of game.
 * The First Human: Real Knife and The Locket in Asgore's Home.
 * The Eighth Human: Stick and Bandage, the items they start with. Also Worn Dagger and Heart Locket in Asgore's Home.
 * Combat Mechanics:
 * Normal Mode: Free movement. Sprouts legs when running away.
 * Snowball Game Text: Try as you might, you continue to be yourself.
 * Owner: The first human and the eighth human.

Aqua Soul

 * Associated Items: Toy Knife and Faded Ribbon found in the Ruins.
 * Combat Mechanics:
 * Foe Attack: Inflicts damage unless you do not move.
 * Omega Flowey: Attacks with Spinning Blades. Heals with Bandages.
 * Snowball Game Text: "Ball" is "Small". You waited, still, for this opportunity... then dethroned "Ball" with a sharp attack. (Patience)

Orange Soul

 * Associated Items: Tough Glove and Manly Bandanna from Snowdin Shopkeeper.
 * Combat Mechanics:
 * Foe Attack: Inflicts damage unless you move through them.
 * Omega Flowey: Attacks with Spinning Gloves with Eyes. Heals with Thumbs Up.
 * Snowball Game Text: You are the kind of person who rushes fists-first through all obstacles. (Bravery/Humility)

Blue Soul

 * Associated Items: Ballet Shoes and Old Tutu in Waterfall.
 * Combat Mechanics:
 * Jump Mode: Jumping over foes and objects.
 * Omega Flowey: Attacks with Ballet Shoes and Stars. Heals with Music Notes.
 * Snowball Game Text: Hopping and twirling, your original style got you through. (Integrity/Kindness)

Purple Soul

 * Associated Items: Torn Notebook and Cloudy Glasses from Gerson.
 * Combat Mechanics:
 * Trap Mode: Moving along Muffet's spiderweb.
 * Foe Attack: Inflicts Sans' Karmic Retribution (KR) poison damage.
 * Omega Flowey: Attacks with Books and Negative Words. Heals with Positive Words.
 * Snowball Game Text: Even when you felt trapped, you took notes and achieved the end of "Ball". (Diligence/Perseverance)

Green Soul

 * Associated Items: Burnt Pan and Stained Apron from Hotland.
 * Combat Mechanics:
 * Shield Mode: Standing still and shielding from Undyne’s spears.
 * Foe Attack: Heals you.
 * Omega Flowey: Attacks with Frying Pans and Fire. Heals with Eggs.
 * Snowball Game Text: Your care and concern for BALL brought you a delicious victory. (Kindness)

Yellow Soul



 * Associated Items: Empty Gun and Cowboy Hat from Bratty and Catty.
 * Combat Mechanics:
 * Shooter Mode: Heart is upside down and shoots bullets upwards toward foe.
 * Omega Flowey: Attacks with a Loaded Gun. Heals with Four-Leaf Clovers.
 * Snowball Game Text: Your sure-fire accuracy put an end to the mayhem of "Ball". (Justice/Temperance)

Trivia

 * Because the ability to LOAD and SAVE the game is associated with Determination, and Determination in turn is associated with red souls, it can be surmised that none of the humans other than Frisk and the first human had the ability to LOAD out of their demise.
 * While not directly stated, it can be assumed that all of the humans that had fallen down into the Underground had been of a young age, although the highest and lowest cannot be determined.
 * Hints to this include the reference Toriel makes when talking about how "each human that falls down here ends up being killed once they leave" and how she's seen it "time and time again."
 * This, coupled with the large box of children shoes that can be found in your bedroom, and the child-like nature of most of the items that have been connected with said humans, all point towards the idea that the Six Children that appeared between Chara and Frisk were in fact of a younger, more child-like age.