Souma is Kendappa's best friend and the last survivor of the Souma clan. Her clan was wiped out by Taishaku-ten army, because of the legend where the Souma clan could grant immortality to one person in their life if they give their blood to that person. Her parents had hid Souma so she did not get killed. After that day, she had always dreamed of taking revenge on Taishaku-ten for murdering her clan. In her travels she passed out in a forest, out of exhaustion, and that was when Kendappa-ou had found her and helped her recover.
The people of the Souma clan were gardeners, so Souma is skilled in making healing herbs and medicines. She uses the Twin Moon Crescents, Sougeppa, as her weapons. They're kind of like miniature sharp crescent-shaped boomerangs that she can throw out. Souma acts as a spy for the group, scouting the area for any of Taishaku-ten's armies who are on their trails.
It's revealed that she and Kendappa-ou are in love with each other.
In final battle Kendappa-ou fought with Souma. Kendappa-ou believed that Souma could not win against Taishaku-ten so she rather kill Souma herself. After mortally wounding Souma, Kendappa-ou committed suicide. Souma wanted to give her blood to Kendappa-ou so she would not die, but Taishaku-ten stopped her from doing so.