Daniel Byrd was Harry Kim's friend at Starfleet, whose place in the altered timeline Kim seemed to have taken.
Daniel Byrd was a friend of Harry Kim who worked at Starfleet's shuttle development facility in San Francisco. In Kim's original timeline Byrd had remained on Earth while Kim served aboard Voyager.
In Non Sequitur, Kim awoke in a reality where he had been denied his posting on Voyager and was working alongside Byrd at the shuttlecraft development center. Digging into records, Kim realized that the two had somehow swapped places in history, and his inquiries into Voyager's classified files soon drew the suspicion of Starfleet security, who feared he was working for the Maquis.