Determination



Determination manifests as SAVE points in the environment, from which the protagonist can restore HP and save their progress. Typically, an encouraging thought or observation of the environment around the protagonist fills them with the drive to keep going. If the protagonist is killed in battle, the game continues from the last point in time they saved from as if they had loaded from the main menu.

Pacifist Route
Determination is a force assumed to be possessed naturally by humans due to the sheer strength of their souls.

A machine created by Alphys in the True Lab was capable of extracting Determination from the human souls possessed by Asgore and injecting it into other hosts, operating under the assumption that Determination was the strength of will that gave human souls the ability to persist after death. Alphys first attempted to inject Determination into dying monsters. Initially, the monsters got up as if nothing had happened, and Alphys planned to send them back to their families, but Alphys quickly found out that monsters could not survive as individual beings when filled with Determination. The monsters injected with determination melted and melded together, their minds and physical matter merging into Amalgamates.

During the initial tests Alphys gathered that she would need a non-monster host to harbor Determination-infused monster souls and break the barrier; humans were out of the question, so she settled for the first golden flower that sprouted in the Underground. Flowey was created from the injection of Determination into a flower that Asriel's ashes had spread on after his death, resurrecting Asriel with his mind intact but with no emotions due to his new, soulless nature. However, now possessing Determination, Flowey could impose his will upon the world through SAVES and LOADS, an ability he would later liken to "playing God."

When the protagonist falls into the Underground, their determination is great enough to override Flowey's, preventing him from using SAVEs and LOADs. Likewise, after absorbing the six human souls, Flowey was able to directly manipulate and override the protagonist's save.

Genocide Route
Undyne appears to be a rare example of a monster with innate Determination. The result of her drawing upon this Determination, when she battles Frisk in this route (or the Neutral Route), is similar to the effects of Alphys' experiments: Undyne is able to survive seemingly lethal wounds, but eventually melts.

At the end of the game, the fallen human describes the determination that they had as being the protagonist's driving desire to continue. The main character gaining EXP, LV, HP, DF, and ATK all correlated with that determination empowering the first human's spirit in possessing and gaining control over the playable character.

Trivia

 * It's unknown if the Amalgamates mimicking game elements such as Frisk's "!" dialogue-box or SAVE points are actually happening and related to the Determination they've been injected with, or simply breaks of the fourth wall intended to catch the player off-guard.
 * It may be that this is also a play on words, as the main power it affords it the ability to reverse death, either the protagonist's or those they may have accidentally killed, hence De-termination.