Frisk is very lazy and unmotivated, they don’t seem to want to keep living but due to their ability to come back to life they gave up on the idea and decided to do something good out of their “infinite will to live” . They seem to avoid fights and try their best to spare any monster on the underground. Frisk gets dragged around by others to do random tasks.
Frisk seems to be a good dodger but prefers to spare any enemy, the hassle to fight is too great.
Frisk the power to SAVE the progress done as well as healing at those SAVE points. Frisk discovered this power after they first awoke in the Ruins
Sans mistook them for Chara and ended up killing them a few times, they soon noticed they could keep dying and thought they were in some kind of purgatory, thinking they had really died. After a few killings Sans questions how many times that had repeated and explained the situation. Realizing his mistake he pretended to take Frisk to the leader of the underground.
Toriel was awfully worried to find a human child on the ruins, wanting to bring her to the safety of her house together with what she thought was a wounded skeleton.