Charles Ryerson was a human Federation citizen living in the late 24th and early 25th centuries.
In 2409, he was discovered by the USS Zife traveling aboard a small craft at relativistic speeds, near Barnard's Star. Due to the time dilation effect, he had aged very little over the twenty years he'd been missing. Meanwhile, another, older version of Ryerson was still living at home with his wife. Starfleet Intelligence arrested both Ryersons, to ascertain if either one was an Undine infiltrator. (STO novel: The Needs of the Many)