In the end of your Genocide Route, you end up in the void and see Someone wearing a Green and yellow Striped shirt there, next to you. She wickedly and evilly smiles waiting for you to get up and talk to her about what you did and done. She says:

Chara: "Greetings, I am Chara. When you first entered the Underground, you awoken me from my eternal sleep. Though I'm dead, I'm still here. Now tell me, Kid, Did you like what you did when you killed the Underground ? Wanna do it again ?"