To get Jade in your party, defeat him with an overkill the first time you face him, and defeat all of his men. Also win by an overkill in the second battle against him. Then, select the "I wasn't really going to kill him" option when it becomes available. In Chapter 4 or 5 get the quest that involves guarding a gold transport. Jade will be there and flee. Go to the guild and look for a letter from Jade. Fight him and he will join your party.