Cosimo's species was a species of non-humanoid lifeforms that existed in a temporal inversion fold located in the space-time matrix.
In 2372, an accident occurred when the shuttlecraft Drake, piloted by Ensign Harry Kim, accidentally intersected one of this species time streams. The outcome was an altered timeline, where "history and events were scrambled a bit," and Harry ended up in a timeline where he never joined the crew of USS Voyager.
Because this species was unsure how or why the accident happened, nor how to return Harry to his proper timeline, they sent Cosimo to watch Harry and to make sure that things worked out right for him. When it was determined that his adjustment to the new timeline was not going well, Cosimo revealed himself, offering him how to find the time stream, and return to his proper timeline. (VOY: "Non Sequitur")