Justice

Justice is the Yellow Soul of one of the Eight Humans who fell into the Underground, their name is unknown. But they are been pretty much confirmed to be armed with a gun, which is now empty. The human with this soul was then captured by Asgore, and then conserved to be used when destroying the Barrier, but this mission then failed. Justice is also the main soul used in Mettaton EX 's fight, where Frisk can activate a button on their new cellphone by Alphys and their soul will be transformed in yellow. Justice, as seen in Mettaton's battle, is an up-side version of a human soul that can shoot from the bottom of the soul, which is now a top.

Trivia

 * It's unknown what is the number of the human with Justice that fell, but it can possibly be one of the last souls that fell down.