In Episode 5, their decision to trade places with Chara and ceasing to exist in the real world seems to be motivated in part by their desire to take responsibility for this, and for screwing over Chara and glitching the timeline in the past. The Atoner: Season 1 begins with Frisk trying to make up for going on the genocide route later episodes shows that their behavior was even worse than this before the start of the series, with them playing around with the monsters' lives countless times, seemingly out of nothing but curiosity.The protagonist of Undertale, and the only soul of pure Determination on the planet.
