Red Justice

Red Justice is the non-binary ship between Frisk and yellow soul from the Undertale fandom.

Canon
They are both fallen children in the underground, but fell at different times. The owner of the yellow soul fell here long before Frisk, judging by Toriel’s words. They also have weapons with them, the yellow soul had a pistol, and Frisk equipped a stick. Along the way, they met many monsters and how they react to them depends on the player's actions. The owner of the yellow soul died in the underground, and Frisk's fate is determined by the player.

Frisk ends up in the underground and often hears mentions of past fallen people and what happened to them. Many monsters say that Frisk will face the same fate at the hands of King Asgore because their soul is the last one to open the barrier. On the way, Frisk enters the store of Сatty and Bretty, who had a cowboy hat and a gun as goods. They admitted that they found these things in the waterfall at the landfill. At the player's choice, Frisk can buy these items and equip them.

While near Asgore's castle, the main character can go into the basement and see the coffins of past humans, among which is the owner of the yellow soul. Later, Frisk enters the throne room and speaks with Asgore, who prepares to kill them like he killed past humans. During the battle, Frisk sees flasks with souls. While the king and the child are fighting, Flowey manages to come up with an insidious plan and capture the souls while no one is looking. Then the flower kills the king and absorbs the souls.

Flowey plans to kill Frisk over and over again using the power of souls. However, after a while, the souls took control and no longer attacked Frisk, but began to help them win. They nursed them back to health and then finished off the flower. After this, it is mentioned that all the souls on the neutral path were missing, meaning that Frisk could use them to get through the barrier. On the pacifist's path, Frisk can enter the room with the coffins again and see that they are all open and there are no more bodies there.

Moments

 * Frisk can use yellow soul equipment during his journey.
 * The yellow soul is one of those who helps Frisk during the battle with Flowey.

Fanon
The appearance of the owner of the yellow soul has never been shown in canon, but in fanon a very popular fan game is Undertale Yellow, in which the appearance and name are revealed and their name is Clover. The developers of this prequel sometimes had a tendency to draw Clover and Frisk together.

Fandom
TUMBLR