1- In the begining of the game you'll see him for the first time inside some really dirty house "looks like a tupe" right beside Aeriths house, he'll be sleeping, the guy beside his house will tell you that he was with the army and he got brain damage... when you go in and try to talk to him aerith will tell you to come back for him later.
2- Go to the item shop in the SAME time in disk 4 buy phenix down + some couple of potions.... go to his house and throw it at him... hell jump out of the bed and join u.... oh ya almost forgot if he asked "who am i" just chose the option "We dont know yet" if u dont chose that option hell get brain damage again.... trust me it worked for me... he can do all limt breaks cloud can do and EVEN more.... his powerfull i got sepiroth final form in one hit.
Hope that helps.
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An Eye For An Eye Makes The Whole World Go Blind- Jesus