In Koihime†Musō she is introduced as a general appointed by the Imperial Court to eradicate Yellow Turbans in other areas. She fought bravely but her number of soldiers eventually ran low and she requests Kazuto for aid. She appears again during the allied campaign against the Tō faction. Shortly after the war she is killed by Enshō when the latter invaded her territory.
In the harem ending she is brought back to life from Kazuto's memories.
She helps the Shoku faction gather soldiers and supplies at the very beginning as Ryūbi's friend. Her kindness is later repaid when she later joins the Shoku faction as a comrade after her territory is suddenly conquered.
She joins the Shoku faction after losing her own territory in battle. In response to Chōun's Kachō Kamen she starts her own superhero act, protecting the people as "Hakuba Kamen" (White-Horse Mask).
A feudal lord who wishes to protect her people from a recent rise of banditry. She loves her white horses. A particular running gag in the series is that everyone tends to forget her name or get her name wrong.
In Shin Koihime†Musō it is revealed that she is a childhood friend of Ryūbi (they also attended primary school together). She briefly gains possession of the Ryūbi's lost sword before pawning it off to Enshō in exchange for food to feed her people. She eventually recognizes the sword's scabbard and regrets her decision. Out of a sense of responsibility she personally accompanies Ryūbi to try to get the sword back from Enshō only to find out that Enshō no longer has the sword. As compensation Enshō cancels Kōsonsan's debt.
In Koihime†Musō she is presented as a friendly and easy-going person. Of all the leaders, she is the only one who treats Kazuto nicely from their first meeting. During the war against the Yellow Turbans, she showed much bravery as a general. However, in the political world she does not hold much presence nor position among other factions. In Chōun's words, while she is not incompetent, she is incapable of being a powerful ruler despite all her valor.
Her plainness is frequently emphasized in the anime, where she is the lord of a small city. Although she really cares for her people and has strong ambitions, she is unrecognized by almost everyone, and those that do know her have a tendency to get her name wrong. Even when she does get the opportunity to shine, she usually let the chance slip away from her grasp. In one instance, she made a grand speech in front of her army, and everything seemed protray her as a great heroic general until she accidentally bites her tongue, ruining her moment of glory.
Her plainness extends to such degree that even though her sword doesn't have a name, it is deliberately labled as "ordinary sword".
However in Shin Koihime†Musō it's revealed that she is an expert in horsemanship, skillful enough to rival Bachō. Together they lead Shoku's cavalry units and during times of peace they oftencompete to see who is better.
Her true name "Pairen" is only revealed in the second VN
Interestingly, her true name is lotus-themed ("White Lotus") despite her not being a member of the Son family.
In the first VN, she, along with Kayū, are the only two girls without any romantic involvements, as they are killed off rather early.
Although, despite not being involved in any romantic events, during her initial meeting with Kazuto she shows tsundere traits.