The Neutral Route is the most flexible in the game. As long as Frisk kills at least one monster, they are guaranteed a neutral route, but the route has many, many variations based on who dies and who doesn't. One thing doesn't change: Frisk must fight Asgore at the end, as Asgore's soul is the key to escaping the magic barrier that surrounds the Underground.[/section]
[section=Pacifist Route]In the Pacifist Route, Frisk decides to kill no monsters on the way through the Underground. If Frisk also befriends every major monster, they will all help Frisk to escape the Underground alive, and in the process, manage to destroy the barrier and free everyone.[/section]
[section=No Mercy Route]In the No Mercy route, Frisk seeks and kills any monster that seeks to oppose them, growing stronger with each kill. This becomes an obsession with strength itself that leads them to butcher the entire kingdom and, eventually, the game world itself.[/section]
See also:
https://undertale.fandom.com/wiki/Frisk