Pacifist:

The longer you survive the battle without hitting anything the more your mercy bar fills up if you hit something the mercy bar will move down a bit

And

genocide:

right after the papyrus fight

Mercy is no longer available to the player as shown in the sans fight concept there

Abilities:

The higher level you get the better abilities

If you go without killing you will gain pacifist run exclusive abilities such as

Basic heal

Basic heal ★

Mastered heal

Dodge

Dodge ★

Poof(teleport kinda like cuphead smoke bomb charm)

Shield

Shield★

Super protector(surrounds the player with spinning shields)

And for genocide and neutral abilities

X slash(from deltarune)

X slash ★

Cross splitter

Rush blade

Rush blade ★

Dark slice

Stomp

Stomp ★

Stunning blow

Lastly there are armor bars below your hp

It takes attacks from the enemy and it acts like a temporary shield that when run out does not replenish until end of battle

And it increases through items that give armor

Max armor bars 5