"Filled with Determination."Underfell Frisk is the Protagonist in Underfell. They are the Seventh fallen child in the Underground. They are currently helped by Flowey to escape the Underground. In Genocide route, Frisk's behavior changes. Everyone started to get scared of them.
True Pacifist Route
After Climbing Mt. Ebott in 201X, Frisk has two decisions, fighting or sparing. Flowey always warns Frisk that the world is 'Kill or be Killed', Trying to make Frisk choose 'FIGHT', Flowey also warns Frisk every time in the beginning of True pacifist route, until Flowey helps Frisk to escape the Underground. Frisk is kind during True Pacifist route, changing everyone for the better, and making them their friend.
Frisk has two decisions, fighting or sparing until Frisk used fighting. Flowey also warns Frisk not to increase their LOVE too much, or else they will become "one of them". Frisk becomes more and more aggressive as the route progresses.
Monsters are free but frisk's soul was sacrificed to Free them and her ghost runs around underground.