SOUL

The SOUL is the very essence of one's being, whether one is human or monster. It plays a crucial role in the progression and plot of Undertale.
In the game, a human SOUL and monster SOUL together can cross the Barrier, while seven human SOULs shatter it entirely.
A SOUL takes on the form of a heart. Human SOULs point down in most cases, and monster SOULs point up.
== Gameplay ==
The player moves the protagonist's SOUL with the arrow keys/joystick. The protagonist's SOUL can move freely in the Bullet Board, and the border encloses the SOUL. In menus, the SOUL indicates the selected option.
During the enemy's turn in an encounter, the protagonist moves their SOUL to avoid attacks. If, due to enough failures to do so, the protagonist's HP reaches 0, their SOUL splits and shatters, resulting in a Game Over. However, during the battle against Asriel Dreemurr, the protagonist's Determination restores their SOUL to full HP if it reaches zero. The SOUL breaks as normal, the text "But it refused" displays, and the SOUL fuses back together. Also, none of Papyrus's attacks can deplete the protagonist's HP, instead causing the battle to end while Papyrus takes them to his shed.
If the protagonist collides with hazards (such as lasers or Undyne's energy spears) in the environment, they enter a brief encounter where the antagonist attacks their SOUL.
The protagonist's SOUL is red. However, SOUL Modes temporarily change its color.
== Monster SOULs ==
The SOUL of a monster is upside down and white. The protagonist sees monster SOULs on multiple occasions, such as when Asriel returns the SOULs to monsters' bodies after breaking the Barrier, or when Toriel or Asgore Dreemurr turn to dust and leave their SOULs behind.
A monster's SOUL is weak and does not persist after death unless said monster is a Boss Monster. Monsters believe that love, hope, and compassion compose their SOULs. Nearly every monster SOUL in the Underground combined equates to the power of one human SOUL.
A monster's SOUL is deeply attuned to their body. This is why a monster becomes weaker when an enemy gains EXP and LV, and why it becomes harder for monsters to fight when they are not inclined to do so. This trait is also why when a monster dies, their form dissipates as their SOUL disappears.
== Human SOULs ==
Human SOULs come in several colors and are stronger than monsters' SOULs. For this reason, human SOULs persist after death.
A human's SOUL is not the entirety of their being, but rather their essence, housed in a physical body. This is why humans are stronger than monsters when it comes to physical attacks. Alphys states that determination is a component of human SOULs, and she was able to extract it from the six fallen humans' SOULs. Due to determination, a human's SOUL is capable of maintaining its form after death. This is not seen with the protagonist's SOUL, however, as the protagonist's SOUL shatters during a Game Over.
It requires the power of (almost) every monster SOUL to equate the strength and determination of a single human SOUL. It seems each additional human SOUL grants exponential power, as 1 human SOUL grants enough power to wipe out a village (as described of Asriel after absorbing Chara's SOUL). Meanwhile, with 6 human SOULs, Photoshop Flowey is able to completely overwrite the rules of battle, and (as stated many times) with 7 human SOULs, a being becomes a God and can do nearly anything, gaining powers such as existence erasure and significant shape-shifting as well as having infinite ATK and DEF thus being immortal. (If Asriel is attacked, it is always a MISS.) It also appears as though a being with seven human SOULs can mimic the magic of any other being, as Asriel after regaining his form as a boss monster thanks to said SOULs also regains his powers as one, such as fire magic.
cs: Duše
de:SEELE
es:ALMA
fi:Sielut
fr:ÂME
ja:SOUL
pl:DUSZA
pt-br:ALMA
ru:ДУША
sr:ДУША
vi:LINH HỒN
:靈魂