I always though that Shuyin was part of Tidus. Tidus is a recarnation of Shuyin. (SPOILER)Tidus never really existed, instead he was just a dream of the fayth, because they remembered Shuyin. Shuyin, however did not die peacfully. That's why we see him in X-2. He thinks he sees Lenne, because of the dress. Lenne and Yuna also look very similar. Which leads me to believe that maybe Yuna is a recarnation. But that probably wouldn't be true considering Yuna was never in the fayth's dream world according to the two games. Tidus was in Zanarkand (the dream world) so that proves this.