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-ablation pump which stimulated her extraction tubules and 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 actress Michelle Agnew. According to the call sheet, Agnew filmed her scenes on Wednesday 19 November 1997 on Paramount Stage 16. Her costume was later sold off on the It's A Wrap! sale and auction on eBay. [1]
She 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.