Cosimo appeared to Harry Kim as a San Francisco café owner but was really an alien who explained how Kim had slipped out of his own timeline.
Cosimo ran a café in San Francisco that Harry Kim frequented in the altered reality he awoke in. He seemed to be an ordinary, friendly proprietor, but he was in fact a member of a species living within the time stream.
In Non Sequitur, Cosimo revealed that Kim's shuttle had collided with his people's time stream, and that the accident had placed Kim in a version of his life where he never joined Voyager. Cosimo told him that he could stay in this new life, but when Kim insisted on returning, he explained that recreating the accident might restore him to his original timeline.