Scharn was an Entharan in a false memory experienced by Seven of Nine due to suppressed memories of being a Borg.
In the false memory, Scharn helped Kovin harvest Seven's nanoprobes. Seven believed that Scharn attached her to a bio-pump that extracted her nanoprobes.
It is unknown if Scharn actually existed. Kovin was proven innocent when Tuvok discovered that weapons testing had triggered memories that Seven experienced as a Borg. (VOY: "Retrospect")