Once a civilian of Zanarkand, 1000 years ago, during the infamous era of the War of Machina between Zanarkand and Bevelle, Shuyin tried to protect a woman he loved---Lenne---by infiltrating Bevelle Underground and trying to activate the strongest machina there, hoping to destroy Bevelle. Change of plans at the last minute, both Shuyin and Lenne died during this event. Shuyin came back, corrupted by his hatred, 1000 years later. His appearance is somewhat similiar to Tidus, although there are, in truth, few differences that betrays their ressemblence.
Shuyin is really screwed up.