At first glance, the Madman seems to be a tribal shaman of some sort in the Juma Tribal Lands, but maybe he left his head uncovered too long in the hot African sun. Just a raving, crazy person. Later, he will seem far more knowledgeable.
When Scarlett first arrives at the Juma Tribal Lands, up the beach from the Anchorage a raving Madman is screaming from a cage, "You don't have the power. No, you don't have it." (He doesn't respond to conversation, and can't be freed – or shut up.)
After Scarlett has completed her confrontation with Princess Chiamaka, the madman will be waiting for Scarlett just outside the entrance to the Necropolis. He will have the gall to say that she looks confused. However, at this point, he can be an interesting fount of information.
Scarlett should avoid calling him a crazy old man, and instead ask for help. In that case, he can provide interesting (though nonessential) background information on Princess Chiamaka and how she was chosen by Victor and became one of the Undead Five. He is also extraordinarily knowledgeable about the Black Grimoire, and even events in Venice, such as the Night of the Knives. (If at all interested in Venetica's story, he's definitely worth a listen.)
After Scarlett has spoken with him post-necropolis, he will wander about the Juma Village, and can be spoken with again. However, if Scarlett has dismissed him as a "crazy old man", he won't offer any information. So don't insult him.