Bo is Duchess Satine Kryze's younger sister. She was in allegiance with Death Watch

but saved her sister and Jedi master Obi-Wan Kenobi, when Maul attacked Mandalore.

After Satine's "death" Bo was never heard from again, but some how Satine survived.
We later found out that Bo and her sister made a switch in order to save the duchess's life. Bo gave her life so the duchess could return to save Mandalore and restore the peace. Satine as no fought alongside Obi-Wan before master Kenobi left to tell the republic what had happened to Mandalore.

(If Satine ever transformed back into her old self, we are not sure, so maybe she stayed as Bo for the remainder of the war.)