Apollyon in Greek, in Hebrew Abaddon.. I one heard him described to me as "Lord of the Pit", or "Gatekeeper or keeper or guard of the gate(s) on that side of it" maybe... Heard him also described to me as "a very dire, but obedient Angel" and/but not Satan, Satan or the Devil is different, and he is not nor will ever be "King of Hell" or a ruler or lord over others in Hell, no one is lord over anyone there for all of them are in equal and severe suffering, anguish and torment continually...
Apollyon/Abaddon is not King there either, there is and are no kings there, he just makes sure nothing goes in or out, unless otherwise specified by God himself, and he is very strong and powerful in his own right, and no one can overcome him, far more powerful than Satan even, but he (Apollyon/Abaddon) is loyal to God, and an obedient and powerful and mighty, and actually quite Holy, Angel of God...
God Bless!