Sai Rin is Shou's twin sister, and renowned as the "manliest woman in Sa Province." When first encountered, she is the head of the Merchant Guild in Koren. After her term of guild leadership expires, she becomes an inventor. When Shuurei calls doctors to treat the disease in Sa Province, Rin designs a smooth knife suited to surgical operations; Rin later designs a centrifuge-like device to separate out sand from adulterated salt.

She fell in love with Yuushun Tei when he was still Ensei's assistant and repeatedly proposed to Yuushun on the Autumn Festival (an event similar to Valentine's Day, when girls give handmade gifts to someone they like). For years, Yuushun had always rejected her. When the Sa conflict ends, Rin announces her last attempt to propose to Yuushun: if he rejected her again, she would give up and wish for his happiness when he returned to Shi province. When Rin approaches Yuushun, he seems to be asleep, so Rin just gives her gift, wishes for his happiness. However, when she turns to leave, Yuushun suddenly grabs her wrist. They are described as newlyweds when they go to Shi Province for the New Year's celebrations.