Yin is a war fan that is shaped like the Yin part of the Taiji symbol. This melee weapon deals massive damage to Fusion monsters. It can be obtained in the China area mission "Emei Trouble".
History
When the war against Fuse reached China, the members of various martial arts schools ended up defending their country from Fusion monsters. Just when it looked like they were about to lose, one aspiring warrior used her black and white war fans to defeat numerous monsters. The citizens, being dazzled by her skills and grateful for being saved from the Fusion menace, named the warrior's black and white fans "Yin" and "Yang" respectively.
Trivia
Yin in Chinese culture is the dark force complementing Yang, the light force.