It's sort of a kind of demon, but it's more of a title. It means "King", so technically, King of Bel translates to King of Kings.

[He'll correct her, but it's done gently. It both is and isn't a kind of demon, and only demons with "Bel" in their names had the right to vy for the throne.]

And yeah, that basically sums up what happened. I ended up consuming the power of another demon called Beldr, and with that, my hat was thrown into the ring for the throne of Bel, because it gave the piece of Abel in me just enough... form, I guess? Anyway, to shorten the next part, demons at that point had invaded Tokyo, and the entire city was put into lockdown, with no way in or out. Enforced by both the government and angels alike. They told us that if we didn't get rid of the demons to pass God's trial, then they'd take away the free will from every human. And worst yet, they told me at one point that they wanted me to use the power of demons to lead mankind as their "Messiah" and spread God's word and law.

[Can you imagine, Weiss? Kazuya could have been an angel...]