Nothing is mentioned about Frisk's backstory prior to climbing Mount Ebott, making their past purposely vague, just like Chara. Reasons for climbing the mountain might be parallel with Chara's hatred for humanity or may simply be the Anomaly controlling Frisk.
Asriel also hints out that there is one reason for Chara climbing Mount Ebott for a very indeterminate intention that Asriel himself says in a saddened and depressive way, hinting out a possible suicidal attempt similar to Chara.
In Undertale, Frisk is controlled by the player. While these actions are controlled by the player and as hinted Chara. It does not mean that Frisk is in the right for committing the genocidal acts of killing others one by one. With the last help being by Flowey with killing Asgore at the end.
Powers and Abilities
Determination Embodiment - This one is pretty iconic to Frisk. They have the red soul and how it ends up slowly being connected with Chara throughout the story being mentioned. They don't all fill in the blanks and it's more of the different various routes being able to try to be able to have this powerful soul that keeps going much like in the Pacifist Route and Neutral Route with various quotes encouraging Frisk to "Stay Determined" and talks about how Frisk is "filled with Determination" throughout various save points.
Indomitable Will - Frisk has the powers to be able to fight enemies and monsters mostly after killing other monsters in the Neutral or Genocide Route where their XP and HP get higher and as they are able to be able to get more and more power.
Soul Manipulation - Frisk is able to control their soul and be able to dodge through certain objects or fight moves from their enemies.
Death Negation - Frisk has the Determination that would be able to get Frisk to come back to life almost escaping Death and getting back up much like in most games like RPG Games where they come back to life from a Save Point.
Companion Allegiance - Frisk whether done on the Genocide or Neutral or Pacifist Route has the ability to be able to have an alliance of some sort. Whether it be all, most, some or one of the characters. They had a close alliance towards Toriel at the start after she saved Frisk's life from Flowey wanting to try to terrorize Frisk and steal their soul possibly killing them. They have been able to create a type of alliance with Flowey and Chara or with some of the other Monsters on the Neutral, Genocide, or Pacifist Route depending on how many fans could speculate and see it through the various options.
Absolute Survivability - Frisk as mentioned earlier has the ability to survive through these circumstances with the power of Determination that keeps them going. When they die they are able to get back from their Save Point whether it would be done through the Genocide, Neutral, or Pacifist Route.
Strong Soul - Frisk has the ability to be able to use their soul to doge out certain attack movies.
Child Prodigy - Frisk has a mind that seems to be really intelligent knowing a lot of stuff possibly guided through by Chara or the Anomaly as the story progresses. It is also possible that it comes from the other characters that guide them through certain puzzles as well and would then would lead to the Anomaly and/or Chara remembering the stuff to be able to tell Frisk about it later on Route by Route done later on in the Timeline(s).
Temporal Reload - Frisk with the accomplishment of the Anomaly or Chara has abilities to have reloads of save files with the exception of the Genocide Route
Event Negation - Frisk has the habilitation to navigate how the story goes based upon their actions that is mainly debated through by the Player known as the Anomaly with the exception of the Reset not working for the Neutral Route against Omega Flowey briefly taking down the Save File or with Chara making it merely as impossible to be able to escape from the Genoicde Route forcing you to read through dialouge with the only exception of Force Quitting the game and then being able to reload the save file or just reset the story as a whole which would lead to the ending with Flowey taunting the player for how cowardly they were like escaping their actions through the Genoicide Route and mentioned how they knew what was happening and encouraging them to want to continue at least or do something as Flowey has been known to taunt the player into trying out the Genoicde Route or Neutral Route with killing the enemies if Flowey finds out the certain Monsters Frisk kills and even mentions their names through the Neutral Route judging Frisk like Sans, except in the Genocide Route Flowey is more into the idea until the end and merely decides to back out and is terrified of Frisk switching tabels and making Frisk merely as more powerful than Flowey thought they would be against Frisk's Determination and actions with underestimating Frisk.
Infinite Resurrection - Frisk has the ability to be able to get back up and be able to try again to go through their mission with the Determination.
Self-resurrection - The self-resurrections go with Frisk's determination willing to continue through and try not to die and keep going.
Aim Dodging - Frisk has the ability depending on the choices made by the player to doge certain attack movies from their enemies. Swordsmanship - Frisk has the ability to use weapons like a sword for the case in the game to fight through other enemies in battles.
Knifemanship - A lot similar to Swordsmanship except a little more iconic with the slice done near the end of the Neutral and Genocide Route and maybe the Pacifist Route from that matter. Basically this is where Frisk has the ability to use the knife to fight their enemies. This has been somewhat of a similar trait with Chara as Chara wanted to wipe out the world and has the option to do so in the Genocide Route and possibly the Neutral Route if actions are done differently.
Gunmanship - This is similar to the Yellow Soul in the Battle with the Omega Flowey from the game's Neutral Route where the game gets closed after the Asgore fight when Flowey kills Asgore and how Frisk fights Flowey. However, regular battles like the fights could have possible guns found and used through the battles.
Melee Weapon Usage - This has to do with a combination attack move that Frisk does. This is usually done through the battles of most of the characters except for some of the battles such as with Boss characters from the Genocide/Neutral like Sans, Asgore, and Flowey and Omega Flowey not being able to show the attack when finally trying to defeat them.
Armor Usage - Frisk would have the ability to choose what to wear giving them armor to wear. Near the end of the Genocide Route Frisk would have the option to wear Locket that was accompanied by the knife.
- Frisk only qualifies as a villain for only the Neutral Route or Genocide Route played through by the player. Their villainous acts with their type and status dependent on the player's choice. Meaning that if the player chooses to do the evil acts, so will Frisk. Potentially making them a pawn of The Anomaly.
- Frisk is Swedish for hale.
- Frisk and Chara share highly similar traits with not just the looks but with their powerful abilities with the Determination along with similar weapon choices and similar story origins falling down a Mountain for a highly vague untold reason that is almost put into more of a mysterious position that would be up for speculation by the fans and audience.
- The Monsters would constantly confuse Frisk and Chara such as Muffet and some of the other characters in the game that specifies a human with a striped shirt. Along with how Asriel at the end of the Pacifist Route and how he would compare Frisk and Chara in a way reminiscing about their memories with Chara during the past.
- Chara has even pointed out a speculation questioning if they are a reincarnation of Frisk on the Genocide Route comparing Health Bars, Level Points, and similar motives depending on the route of the Player's choices done in the story's timeline.
- Frisk's role is more of a Neutral Evil as they could have the option depending on the Anomaly as the player practically controlling Frisk's actions could be able to have the option to not have the genocidal actions taken at all or change their mind getting back to continue out the heinous acts while eventually being mocked on the Pacifist/Neutral Route ending for evading the Genocide Route by Flowey similar to how Flowey talks about to the Anomaly referring to them as Chara or whichever name they had chosen encouraging them not to reset and give Frisk and the other characters the happy ending if the Genocide Routes haven't been acted upon and there wasn't any resets done to escape from it the route.
- In spite of Frisk's appearances remaining the same way most of the time. There are some narration and dialogue from the other characters regarding how evil and frightening Frisk can be with pointing out their less human-like look that Sans would see as unrecognizable to that point, or with Papyrus pointing out the dust around them and trying to suggest for them to take a different path as he may have figured out that the monsters were killed and that Frisk was the one that killed them, or how characters refuse to want to fight or face Frisk and would act more anxious and scared, or how some characters like Mettaton would just be in disgust all being reasonable reactions and responses towards what Frisk has done.
- There are some glitches where some different attacks from different weapons or fighting skills don't get used through the brief end of the Undertale Neutral or Genocide Route Battles against Sans, Flowey, Asgore, and so on. A lot isn't mentioned on why the attacks aren't using the combination attacks through the Genocide when fighting and then eventually killing Sans, or when killing off Asgore, and/or Neutral Route when choosing to kill Asgore where they use the similar attacks Toy or the real knife from the Heart-Shaped Box.