Her name is never mentioned in any manga or anime episode or chapter or OVA, but "Kouran" was only given on the light novel Shoryu Den.
Kouran is never seen in the face directly, so appearance is very hard to describe. Matching descriptions are long hair, few inches shorter than Hiko.
Although Kouran never appears long in any of the installations/chapters/adaption or even in the light novel, Kouran is described by Hiko as a woman who is loyal and undeceiving. This was also half the reason the fight heated up more, Hiko even says that "even if it was just once...even just once she let another man touch her". Kouran is also suspected to be very kind and is seen most of the time smiling.
StoryChichiri announced his and Kouran's engagement to Hiko first, telling him that he would like Hiko to be the first person to know. Chichiri was hiding on a tree, holding a kind-of gift for him to give to Kouran and was wearing a big smile. But when he turned to look, Hiko and Kouran were already kissing, and when Chichiri saw this, he was very furious, and Kouran apologized and said that she can't be with him like he planned.
A flood followed and Chichiri held a knife over the neck of Hiko on a small rock-cliff and Chichiri told Hiko to give Kouran back. Suddenly Hiko slipped of the rock cliff and fell to the powerful waves and water currents the flood produced, and Chichiri held on to Hiko's hand. Chichiri let go, and another moment he planned to save him, but a tree crashed on him and gouged out his eye.