1.Go to any forest in the world and wait until you come across her.
2.when you find her you will have to fight her when you do beat her.
3.Once you have beaten her do not go to the save point which will be there.
4.She will be lying on the ground go over and talk to her.
5.keep talking to her and answer the questions she asks like this:
after this she should join your party.you can now defeat her father in Wutai for Leveithan or whatever it's called materia
Go to the mansion in Nibblehiem and open the safe using this code [right to 36,left to 10,right to 59 and right to 97] Then fight the boss and get the materia then go down stairs into a room full of coffins to the left of Cloud near where Sephiroph is and open his coffin [the one in the middle] and talk to him then leave and go back in again then he will join your party.
He is one of my favorite charactors and will come in handy.