Hoshi no Oujo: Uchuu Ishiki ni Mezameta Yoshitsune

Her brother Shanaou was killed by a member of the Heike clan.

For vengeance and to restore the Minamoto clan, the protagonist Sakurako vows to destroy all the Heike.

Introducing herself as Shanaou in place of her late brother, along with her childhood friend Kisanta she goes off to Kurama Temple for training.