The Soul (Utdr) the player controls in Undertale and Deltarune, depicted in red by default. In Undertale, it can become Blue, Purple, Green, Yellow, and Blue/Red (Orange too in Deltarune) depending on the gameplay style of the fight.
In both games, the Red Soul is explicitly identified with the player, with Frisk (Undertale) being revealed as a distinct character at the end of Undertale, while Kris (Deltarune) can rip you out and fight with you as a distinct character in your own right.