So in short it the pyrefiles that once used to give Shuyin substance is sucked away by Yevon while summoning the gravity spell into dream Zanarkand to give Tidus substance since:
Sin isn't an Aeon anyway, actually. Sin is just a whole lot of Pyreflies held together by Gravity Spells.
And for the looks and character it is due to:
Pyreflies are known to react with our
thoughts and preserve them as illusions. They are related to fiends, aeons, and even Sin, but exactly how remains unclear.
But look at this, if Tidus is form by pyrefiles which once used to form Shuyin, dun you think Tidus will have some memories or some illusions that is inter-related with Shuyin. Like when Yuna used the songstress' dress sphere, she inherited some of Leene's emotions(thoughts and memories too? Sorry, I can't remember..) and started having dreams like being shot dead.
Of course since pyrefiles and sphere is two different things but surely Tidus might have some images of Shuyin since they are so closely related but one can also argue that Yevon wash away all those stuff when Tidus is in Dream Zanarkand. But do anyone think that it will be possible to at least gain back some thoughts or images of Shuyin when Tidus is out of Dream Zanarkand.
Just my little silly thoughts though...
