(covers information from several alternate timelines)
Cosimo was the identity assumed by an alien from a temporal inversion fold in the spacetime matrix, who took the form of a Human owner of a coffee shop in San Francisco, in order to interact with Ensign Harry Kim, who had inadvertently created an alternate timeline in 2372.
Explaining to Harry that history had changed when Harry's shuttlecraft intersected one of his species' timestreams, he instructed Harry on how to locate the timestream again, informing him that, if he were to fly into it and recreate the same set of circumstances as happened during the initial accident, he might be able to repair the damage to history, but counseled Harry that he could not guarantee that he wouldn't make things worse by doing so. Deciding that he needed to try, Harry used a stolen runabout and, with the help of the alternate timeline-counterpart of Tom Paris, was able to recreate the accident and undo the alterations to the timeline. (VOY: "Non Sequitur")