Hmm, so Shuyin is...?

[Renmiri]

Tidus did exist though, in the same way as summoned Aeons exist. You could say Tidus actually is the same as an aeon, just not as powerful, and not aware of it himself (not until the end anyway). Also, Yuna didn't look similiar to Lenne. Shuyin was fooled by the dressphere, which projected Lenne's personality onto Yuna. I think that makes sense, as Shuyin was basically a ghost. I don't think ghosts are fooled by appearances, that's why he instead saw Lenne's 'spirit'.

Good catch! Shuyin's being fooled by the ressemblance never really sounded too credible to me.

Agree 100%, Tidus and DZ are basically Aeons

[~SapphireStar~]

From the gamewriters point of view, he a was a way to motivate Yuna to become a sphere hunter and run all over Spira, looking for clues about him.

From the story's perspective, his role is a little more unclear. As you mentioned his feelings of hatred and despair at his death got trapped in the Den of Woe and were perpetuated by the Pyreflies. That's why anytime he possesses someone, Pyreflies enter their body. In a sense, he's just a ghost.

It's a very Japanese conception of ghosts though. His traumatic death imprinted his negative feelings onto a place, where they have been perpetuated for a millenium by the Pyreflies. That's the sort of idea we see in movies like Dark Water or The Grudge.
As for his connection to Tidus, I agree he might have been dreamed up by the Fayth with Shuyin in mind. Surely many of the entombed Fayth knew him personally. It may partly explain how Tidus became smitten with Yuna. But I doubt he had any of Shuyin's memories.

I like this also, I agree.