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.
Kovin was proven innocent when Tuvok discovered that weapons testing had triggered memories that Seven experienced as a Borg. (VOY: "Retrospect")
Scharn was played by Michelle Agnew.
Scharn was named by Bryan Fuller
. He based the name on the fact his mother had the maiden name "Scharnhorst". (Information from Larry Nemecek)
It is unknown if Scharn actually existed or was a construct of Seven's imagination.