Leto wasn't really a person in the traditional sense. His consciousness was more of a council of ancestors that was dominated by a single strong personality. That personality wasn't named, but it was implied to be an ancient Egyptian Pharaoh (my interpretation and others agree).

Leto II, Alia, and Ghanima were all pre-born and all found a different way to survive becoming consciousness before having a strong central conciousness. Leto II I described above, Ghanima cured herself by doing the memory trick that convinced herself that Leto II was killed by the tigers, and Alia let the Baron Harkonnen take her over.

From Children of Dune:

"Well and good, cousin. She asked me if I were Abomination. I answered in the negative. That was my first treachery. You see, Ghanima escaped this, but I did not. I was forced to balance the inner lives under the pressure of excessive melange. I had to seek the active cooperation of those aroused lives within me. Doing this, I avoided the most malignant and chose a dominant helper thrust upon me by the inner awareness which was my father. I am not, in truth, my father or this helper. Then again, I am not the Second Leto."

"Explain."

"You have an admirable directness," Leto said. "I'm a community dominated by one who was ancient and surpassingly powerful. He fathered a dynasty which endured for three thousand of our years. His name was Harum and, until his line trailed out in the congenital weaknesses and superstitions of a descendant, his subjects lived in a rhythmic sublimity. They moved unconsciously with the changes of the seasons. They bred individuals who tended to be short-lived, superstitious, and easily led by a god-king. Taken as a whole, they were a powerful people. Their survival as a species became habit."