SOUL

The soul
The soul is the very essence of ones being, be it Humans or Monsters.

Monster soul
The soul of a monster is always white in color and is shown in multiple occasions, such as when Flowey takes all the souls of the monsters and is recreated as Asriel Dreemurr, or when you kill Toriel or Asgore Dreemurr.

The soul of a monster, very much unlike a human soul, is weak and does not persist after death, unless said monster was what one would call a Boss Monster

Human soul
The soul of a human can come in several colors, and is much more powerful than a monster. Being shown, is when Flowey absorbed 6 human souls and became almost godlike. However, unlike a monster soul, the soul of a human persists long after death.