Your relative is the defendant. Your relative must appear or risk a default judgment against him. Your relative's defense is that he was acting as your agent and is not responsible for acting on your instructions.
You cannot speak for him or represent him but he can call you as a witness and you can testify in his behalf.
He can then ask for a dismissal.
The plaintiff then will either have to file a lawsuit against you or ask permission of the court to amend his complaint. What happens after that is anybody's guess.
I noticed that you posted your question on two other legal websites. I doubt that you'll get much different advice on them.
Thanks for the reply. Yes he plans on calling me as a witness. What do you mean a lawsuit against me? An appeal after a judgement is made?