Chara

Chara is the overarching antagonist of UNDERTALE, being the overarching antagonist of the Neutral and Pacifist Routes, and one of the two main antagonists alongside Sans, (although he’s a hero) of the Genocide Route.

In All Routes

 * Could be sadistic implied by the fact that when Asgore was poisoned, they laughed.
 * Only appers after killing every monster, hinting they are genocidal.
 * It's implied that they to want to destroy humanity.

Only Genocide Route (Most Likely Non-Canon)

 * Seeks to destroy and Reset the whole world.
 * Kills their former best friend for no reason.
 * Even if you stop doing the genocide route, they will still appear after every game unless you delete the game and all progress and then re-download it.
 * They likely crossed the Moral Event Horizon.

Why They Were Rejected

 * They lack Moral Agency and is left too ambigous to be even Near Pure Evil.
 * Depending on the route you take, Chara can be either a Hero or Villain.
 * They're Grey Zone and therefore cannot be Near Pure Evil in the first place despite potentially crossing the Moral Event Horizon.

Trivia
Chara and the King of Spades are the only two Undertale/Delturune characters to be rejected for Pure Evil.