Soap (せっけん, Sekken) is an item in the first Suikoden. Soap is a unique quest item, needed to obtain Sarah. Afterwards, it can be used in your bath at Toran Castle.
Suikoden
When Tir first met Sarah in Kirov she agreed to join the Toran Liberation Army under one condition: That he procure her some soap. To do this, Tir had to embark on a trading spree within the city, purchasing Sugar in the item shop and exchanging that for a Yardstick, the yardstick for Salt, the salt for Soy Sauce and, finally, the soy sauce for soap.
When Tir brought the soap to Sarah, it turned out she had some extra and so didn't take Tir's.