use this combination in the old shinra mansion in niblheim right 36 left10 right 59 right 97 then get the key to the basement after you fight the lost number inside the safe go to the basement open up the door whare you find ying yang to fight in that room open up the door down there then talk to him talk to him until he says let me sleep then go on your way upstairs and he will follow you and ask to join you