Ok, I'll give you the answer now. I'll admit that it was very obscure, but I thought someone ought to have got it after I gave out some clues. The answer is character's birthdays and the months of the year they are in. It works like this:
My first yeilded none, (no birthdays in January)
My second, two, (both Aeris and Cid are born in February)
One strong of heart, the other flew,
Third and fourth, the same as the first, (no birthdays in March or April)
To a lover the fifth gave birth, (Tifa was born in the fifth month, May)
Sixth and seventh, no reward, (no birthdays in June or July)
Eighth lost himselft, but found a sword, (Cloud was born in August)
Nineth went by without a sound, (no birthdays)
Tenth was tricky, sneaky, proud, (Yuffie was born in October)
Eleventh was dark and mysterious, (Vincent was born in November)
Twelth knew who was boss, (Barret was born in December)
There are two more but noone knows,
Where they start, or where they go. (Cait Sith and Red XIII's birthdays are unknown, so no one knows when they sarted life)
I guess that leaves it open for someone to post their riddle.
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