Oh ok so maybe it's Cloud's Ultima weapon since his name starts with a C? If you don't see Ultima Weapon just wait until Cid's theme is playing on the world map is I think when he first appears.

Yuffie's ultimate weapon starts with a c, conformer, but that doesn't seem to fit the riddle since as soon as you get the sub you can get it, and you don't have access to the area beforehand.

Maybe since Cid starts with C, it's his ultimate weapon. I think the old guy gives it to you by the rocket. Oh yeah you have to talk to him a bunch of times so it kinda fits the riddle, and also if he gives you the Yoshiyuki, you have to wait until there's no rocket to "see," then you get the Venus Gospel.