Like d£v!l'$ ph0£n!x said:It depends on a variety of things.

-How many times you summon him...

-How much Gil you pay him...

-What answer you choose to the question his fayth asks...

-What attacks he used last time...

-How much Gil ypu payed to recruit him...

I'm not sure how to get Zanmato,'cause I don't even waste my time fooling around with it.

Just check GameFAQs.