In the 2015 RPG "Undertale", which character do you first encounter after falling down into the underground?

In the 2015 RPG "Undertale," the character you first encounter after falling down into the underground is Flowey. This encounter with Flowey is significant because it sets the tone for the game, as he initially appears friendly and helpful before revealing his true nature as a malicious and manipulative entity. Flowey serves as the primary antagonist throughout the game, making this first encounter pivotal in establishing the game's themes of deception and moral choices, ultimately setting the stage for the player's journey through the underground.