Determination

The Determination is the SOUL of the Eight Human who fell into the Undergound, which is Frisk. And also the Soul of the first human, Chara. The Determination is possibly only "owned" by Frisk and Chara, there might be other Humans out there who have Determination too. But this is currently unknown, and it will most likely never be discovered.

Trivia

 * Determination may also be Kris's trait from Deltarune, as when they are in a battle, their Soul is red. Like Frisk and Chara.
 * Canonically, The Determination is the first soul that never got captured by Asgore Dreemurr.