Stats

Stats are aspects that affect encounters. Most stats increase as the protagonist's LV rises, although other methods can increase the stats.

HP
HP (Hit Points or Health Points in most RPGs) is the level of Endurance that determines the damage a monster or the protagonist can take before dying. The protagonist's maximum HP starts at 20 and only increases when their LV does. Resting at the Inn or ASL resort Bedroom fully heals and raise the protagonist's HP 10 points above its cap. HP can also be restored through consumable items or interacting with a SAVE Point.

An attack hitting the receiving party lowers their HP. The loss is dependent on the attacking party's Attack and the party of receipt's Defense. Upon reaching 0 HP, the protagonist's SOUL snaps in half and shatters, sending them to the GAME OVER screen. If a monster's HP reaches 0, they disintegrate into dust and reward the protagonist with EXP and GOLD.

HP has been abbreviated for Horsepower or Hit Poodles from (Insert Aaron) and the Dog Salad's info Checks respectively.

AT
AT (Attack or ATK traditionally) determines the damage output of the attacking party. The protagonist's base AT increases with LV and equipping a weapon. AT can be changed temporarily during battles with ACTs or consumables.

When the protagonist's AT is high enough, the protagonist can spare monsters without ACTing. This rule is true even of monsters that have a non-traditional sparing method: Undyne can be spared immediately, and the colored name and everything else shows if the total AT is 20442 or higher (though the game softlocks after doing so).

DF
DF (Defense or DEF) determines the damage input for the defending party. The protagonist's base DF increases with LV and equipping armor. DF can be changed temporarily during battles with certain ACTs or consumables.

EXP
EXP (Execution Points) are gained by killing monsters. When the protagonist accumulates enough EXP, their LV increases. In the Undertale Demo, EXP is gained by attacking and sparing afterward and not necessarily killing.

The acronym is designed to mislead players, as in most games, EXP stands for Experience Points. If the protagonist "kills" Chara, they lose one "experience point," which does not affect their EXP and serves as an early clue that EXP are not Experience Points, but a different stat entirely.

LOVE
LV or LOVE (Level of Violence) increases HP, AT, and DF when it rises. Sans describes it as "A way of measuring someone's capacity to hurt." The protagonist starts at LV 1 and can rise as far as 20 in-game. LV can be set arbitrarily high or low via file editing.

INV
INV (Invulnerability) determines the number of frames an attack cannot hurt the protagonist after they receive one. It is a hidden stat that the protagonist can only increase through the Cloudy Glasses, Torn Notebook, and Temmie Armor. The former two items explain INV in their Info check.

Invulnerability is set to 0 during the fight against Bratty thanks to her KARMA ability. The Cloudy Glasses and Torn Notebook will only increase INV by small fractions of a frame when battling Bratty.

SPEED
SPEED determines the velocity the protagonist's SOUL can move around the Bullet Board. It is a hidden stat that the protagonist can modify temporarily only through Sea Tea, certain ACTs and in a fight with (insert Vulkin) using the Thunder attack.

Murder Level
MDR (Murder Level) is an entirely internal statistic that tracks the protagonist's progress through the Genocide Route. This statistic begins at 0 on every playthrough and is reset by a simple Reset. The murder level is accessed a few times in-game, notably to affect types of bosses encountered and boss activity, to decide whether or not the protagonist battles Sans, and to find the correct Neutral Route ending to play depending on the protagonist's choices.

Flags in the protagonist's save file are accessed and used to determine the choices the protagonist has made, and whether or not those choices fulfill the requirements for a complete or aborted Genocide Route attempt. Except for the murder level override, each step increments the murder level only if all of the requirements preceding it have been fulfilled. For example, if the protagonist has killed Undyne but did not exhaust the Ruins kill counter beforehand, the murder level remains at 0.

If the protagonists murder level is 16 or higher, in Final Corridor, they fight Bratty.

The line number listed below corresponds to the line number in the protagonist's save file, i.e. 31 greater than the flag number. The murder level override is checked after this, ignoring the above calculation: The protagonist battles Bratty if the murder level is 16 or higher.

Trivia

 * When the protagonist enters a battle with an LV of 20, their AT and DF automatically sets to 30 and their maximum HP to 99. This change is never seen in-game because the only instance of the protagonist's LV getting to 20 is after their battle with Sans, who is the final opponent the protagonist can practically fight.