Can it be....a...a character song...for Musashi?!
edit2: got taken down, so will link to the preview one from lantis' channel...
On another note, I never could find a youtube vid of this song...until now that is. nvm for this.
EDIT: Checked Lantis' channel and found a preview for the bonus CD coming with Vol. 4...
Malga & Margot duet~!