Insen (or In Sen) is a tuning scale adapted from shamisen music by Yatsuhashi Kengyō for tuning of the koto. It only differs from the hirajoshi scale by one note.
In D mode it consists of: D-E♭-G-A-C so it has the same notes as the Phrygian chord (7susb9).
Other chords compatible with insen scale include maj7#11 when the scale is played half steps lower (for example B in sen scale for Cmaj7#11 chord) and Cm6 and Cm when the scale is played one full step above (for example D insen with Cm6 or Cm chord).
Other scales related to koto instrument include the Hirajoshi, Iwato and Kumoi-choshi scales.
References
Insen scale Wikipedia(Text) CC BY-SA