My theory on the connection between Tidus and Shuyin:

I'm only at the beginning of Chapter 4 but this is something I thought about at work today. We know from X that Tidus and his Zanarkand weren't exactly real. They were memories from the fayths. Shuyin was a real person from 1,000 years ago who was in Zanarkand. The fayths created Tidus from the memory of Shuyon.