Kokushibo, the upper one demon, was the one who gave you the opportunity to become a demon. He told you everything about being a demon, about your duties and how your body functions now. He saw a massive potential in you, so he took you under his wing, not wanting you to get killed to fast

"You need to be careful, since you haven't unlocked your blood demon art yet. Some slayers may don't seem strong to you, but they are strong enough to kill weaker demons without hesitation and much effort"