|Japanese name:||趙雲 子龍/星（せい）|
|Nickname:||Sei, Butterfly Mask|
Chōun is a mysterious girl who appears calm, collected, and very powerful. She is the second person to join the group. She loves to tease Kan'u and other women frequently. She also assumes the identity of a hero of justice named Butterfly Mask, hence wearing a Butterfly mask.
In Shin Koihime Musou, Chō'un (in Kachou Kamen's form), saves Ryūbi from bandits with her secret technique, making Ryūbi develop a crush on Kachou Kamen. Chōun asked Ryūbi who saved her, and she 'innocently' said good things about her (Kachou Kamen is super cool, and noble), raising Chōun's mood high. Chōun is also scary when she's smiling (Sui will have nightmares about it and Shion won't let her daughter sees Chōun's smiling face). Chōun even continues teasing Kan'u in more ridiculous ways (like an embarrassing introduction of Kan'u to the bandits who attacked Ryūbi - and Chōun gets a lump on the head as a result).
In Koihime Musou's OVA, Chō'un (as Kachou Kamen) appeared and swam, but Kouchuu, who acted as the school's doctor, said that its okay, since Kachou kamen's appearance looks like Chō'un, and she immediately tied up and put into an unused class, until night and no one seems to notice her missing (even her daughter, who asked about her whereabouts before).
Visual novel version
When Kan'u was fighting the Tō army, she fought alongside Chō'un for a while; when this war ended, Chō'un met Kazuto and took a liking to him, promising to join his army later. She joins alongside Kōchū after the battle with Enshō. She has a strong obsession of menma. She also has a habit of running around the city as the masked hero of justice 'Butterfly Mask'. Strangely, no one other than Kazuto and Shuri can recognize her true identity, despite only wearing a mask.
Voiced by: Emi Motoi (visual novel, anime) (credited as Nana Nogami in the visual novel)