Ethan was a young boy who claimed to be a prisoner in a Romulan laboratory along with Commander William T. Riker. It was later discovered that he was the key figure in a holographic simulation created by the alien Barash on the planet Alpha Onias III.
"Ethan" was in fact Barash himself, who was merely seeking interaction with Riker. He had been isolated for several years in a holographic environment that responded to his thoughts and attempted to fulfill his desires. The only thing it couldn't do was provide him interactions with other real beings.
During this same visit to Alpha Onias III, Barash had also appeared as Jean-Luc Riker. In the scenario created by Barash, Commander Tomalak introduced Riker to "Ethan," claiming that he and his Romulan colleagues had used his image to augment their program. (TNG: "Future Imperfect")