Souma is Kendappa's best friend and the last survivor of the Souma clan. Her clan was killed by Taishakuten and her parents were beheaded right before her eyes. Her parents had hid Souma in some bushes so she did not get killed. After that day, she had always dreamed of getting revenge and beating the crap out of Taishakuten, and she set out traveling by herself. She passed out in a forest, out of exhaustion, and that was when Kendappa-ou comes a-riding into the forest on her horse, and happens to see Souma laying there unconscious. The people of the Souma clan were gardeners, so Souma is skilled in making healing herbs and medicines. She also has weapons, the Sougeppa, or Twin Moon Crescents. They're kind of like miniature crescent-shaped boomerangs that she can throw out, and they can cut off people's heads and stuff. Souma also makes a good spy. She's fast and jumpy, likes to hide up in trees, and seems to disappear or appear all of a sudden in front of the group to bring them news of any of Taishakuten's armies who are on their trails.
It's revealed that she and Kendappa-ou are in love with each other.
Souma is killed by Kendappa-ou. Kendappa-ou believed that Souma could not win against Taishakuten so she would rather kill Souma herself. Souma wanted to give her blood to Kendappa-ou (there was a legend where the Souma clan could grant immortality to one person in their life if they give their blood to that person) so she would not die but Taishakuten stopped her from doing so. The two bodies were put next to each other.
(Source: Nishi's RG Veda Gallery)