First Human

'The First Human' refers to the first human child to ever fall past the Barrier on Mount Ebott into the Underground. This person is in fact who the player is naming at the start of the game, and not the controllable character who is played throughout the entirety of Undertale.

Personality
While initially characterized as innocent and pure by Asriel and others (primarily while they are talking to Frisk but using the First human's name), Asriel admits after beating defeated in the Pacifist route that the first human climbed the mountain for 'bad' reasons, and similarly had a dark outlook of humanity. It was them that wanted to attack the humans when they opened fire on Asriel/Them. The first human is a very cruel and selfish person, who wants to destroy most everything, although they apparently held a very close bond to their family, specifically Asriel. In the Genocide Route his character is more revealed as having been the 'determination' by which Frisk had killed everything and everything one, and his desire to continue with that plan as he moves past the Barrier.

History
This first human to fall into the Undergound after the war between humans and monsters was taken in by Toriel and Asgore as a second child, whom they treated with respect equal to their genetic son, Asriel. Asriel quickly became friends with the fallen child. They spent much time together, and talked of their dreams- both of them wanted to venture to the Surface and play upon the Mountain. This all occurred until the human became very ill. The humn told Asriel as he was getting worse and worse that he had one wish- to see the Golden Flowers of his hometown. Unable to leave the Barrier, Asriel could not help his brother and friend achieve this. Eventually, the human died, and grieving, Asriel took the soul of the human within himself become incredibly powerful. Having the soul of a human and a monster, he was strong enough to pass the Barrier, and he left the Underground. He returned the fallen child's body to the town from which he came, so that his body could once more be with the Golden Flowers of the Surface. However, he was was attacked by humans that had thought that he had killed the child. Asriel, being controlled by both his own soul and the child's struggles whether to destroy his attackers, or walk away and return to the Underground. The first human urges the body to attack, but Asriel overcomes this and decides to flee. After returning to his parents, he died from injuries sustained above ground. After this, the soul of the first human became a spirit that dwelt in the Underground, waiting for the humans that it could challenge and attempt to possess. This is dilemma that Frisk, the playable character faces. The struggle between the three endings, the Neutral, Pacifist, and Genocide depend on the success or failure of the first human to manifest control of Frisk by way of the player.