All the characters, even the ones who aren't the main bosses have so much personalities. And the plotline of this story is so well crafted. And when the game is over it still leaves you with so many questions!It has a bittersweet ending and I LOVE IT. Undertale has 3 ending: Pacifist, Neutral and Genocide. And those are the major ones. But different gameplays consists of different characters. Sometimes you might even meet W.D Gaster :D
I totally recommend this game. It has all types of characters. Though it isn't for everyone, you should still try it out!