The dogmatist
The dogmatist is one of Kant's two main implied adversaries. While he isn't always responding to specific dogmatic philosophers, this position is always lurking in the background as something Kant is criticizing and responding too Dogmatic philosophers (like Leibniz) make big, speculative claims about the nature of the transcendent entities—God, the Cosmos, the Immortal Soul—that by definition can't be verified. The dogmatist tends toward the religious, poetic, and speculative, and has grand ambitions for philosophy.
The skeptic
The skeptic is Kant's other main implied adversary. Represented especially by Hume, the skeptic is the opposite of the dogmatism. He always wants to deflate other people's claims. You say that fire causes water to boil, but how can you be sure? How do you know it is the fire, and not some other invisible force causing the change? And how do you even know that you did see fire, or even water? Isn't it possible that you dreamt the whole thing—after all, don't we all commonly have the experience of thinking we are observing something, only later to wake up and realize it was all a dream? The skeptic doesn't believe that philosophy can ever arrive at certain knowledge, and so his main goal is to curb the ambitions of philosophers and challenge their claims. Just as Kant had to reign in the over-reaching ambitions of the dogmatism, he will have to curb the skeptic's corrosive doubt and establish a secure basis for philosophical knowledge
