The Gate Keeper is a bizarre little man wrapped up in a cocoon who would drop down from the ceiling to allow the Onimusha heroes into the Dark Realm. His name is Minogoro and he is a member of the Mino clan. He has appeared throughout the Nobunaga trilogy serving his usual purpose, though he does not appear in Onimusha 2: Samurai's Destiny.
In Onimusha: Dawn of Dreams, Minokichi is his son and successor. Apparently, after the events in Onimusha 3, the Genma, angered by the Gate Keeper allowing the Onimusha to pass into the Dark Realm, hunted him down and killed him.