Frisk was a happy child before their parents were killed in a car accident when their mother attempted to drive while doing their taxes, fixing their make-up and getting a blowjob. Months later, they fell "accidentally" through a hole and ended up in the underworld. Apparently, they wanted to commit suicide for reasons unknown (that it might have something to do with their severe depression).
Once in the Underground, they quickly became possessed by the ghost of Chara who was teleported there from 700 years in the past. This forced Frisk to kill some people in their first run before resetting and doing a True Pacifist Run. There exists a universe where Chara wins and does a Genocide Run, but it is best that I don't give You nightmares.
Having the soul of True Determination, they have the ability to load, save, and reset the game of UnderTale. As such, they get pretty much whatever the Hell they want out of the Underground. They won the fight with Asriel Dreemurr (who had obtained the equivalent of seven human souls) because their soul refused to die, and Asriel finally decided to give up peacefully and even destroy the barrier for them. In terms of raw strength, however, they are quite powerful since Undyne (who had much better armor) and Asgore (who had the absolute best armor in the game) still fell to them (even if Frisk had to reset once on each fight). However, their power level depends on their Level of Violence which varies depending on how many monsters they've killed. Their base power level is 30, and it increases by 10 with each level up until level 10. After that, it increases even more quickly each time their level of violence increases, going from 10 to 20 to 30 to 40. At level 18, their power level is all the way at 450, and at level 19, it becomes 500. At level 20, their power level is unknown because their ability to manipulate time is too great. Below is a table that attempts to elaborate on this, but it mostly just takes up space on the wiki.
|Level of Violence||Power Level|
|6||80 (starts increasing by integers of 20)|
|8||120 (starts increasing by integers of 30)|
|15||330 (starts increasing by integers of 40)|