dude why the hell would madara get scared when he sees the face of a girl named rin that person should be strong strong enough to get madara wet his pants
lol see your way of thinking is too typical. You don't go the extra mile. There are plenty of ways to get scared, it doesn't have to be someone who is more powerful than you. One example, you can get scared by knowing that your weakness will be revealed if you take that step.
Note: I did say that this is if madara/mask man/tobi is obito and this is only my opinion/theory.
Kakashi gaiden: we see obito who cares a lot about Rin. He loves Rin a lot so much that he would risk his life to save her. But Rin didn't love Obito, she loved kakashi.
Later in the chapters we find out that Rin died but we don't know how or when she died. I don't even think kakashi knows. We don't know what happened to Rin. It's a big mystery. Eventually Rin will have to take part in this whole story line again in some way because she was the member of team 7 led by Minato. She is an important character that kishi is hiding some information about her because it's necessary for the big surprise since kishi loves to give huge surprises to everyone.
Why would madara be possibly scared of Rin?
We know that madara was severely damaged when he fought the first hokage so his body was practically done for. He needed another body. A special Uchiha body and not a body of a simple/normal uchiha body. By saying "a special Uchiha Body", I am saying a body with special chakra enough to make a sharingan go to the next levels. Not all sharingan users can take the sharingan to the next level which is MS. They need special chakras. At that time only worthy people to take body of were shishue and obito. I am sure someone like madara would be able to know who is special or not. Since obito was crushed by bunch of rocks and thought to be dead by everyone in konoha and uchiha. It was a perfect body to revive and fix without bringing any suspicion from konoha and uchiha clan. No one will look or be worry about a dead body. At that time madara needed to be as discrete as possible. He didn't want anyone to find out about him.
so somehow madara managed to revive obito and fix his severely damaged body with the help of zetsu. Madara then used some kind of forbidden jutsu to transfer bodies and in doing so he suppressed Obito deep inside of him. He have seen that this can be done. Oro changed bodies every three years. So it's possible, I am not going to limit myself by saying that only oro was able to do this type of thing. I am sure someone as knowledgeable as madara would at least know how to do this trick.
Having Obito suppressed inside of him is important for him. He doesn't want obito to take over his own body. Madara wants to have full control over obito's body so he can carry on with his plans. If obito were ever to be awaken, madara would lose control over that body and it would only be bad for him.
I am pretty sure if Obito heard Rin's voice he would do everything he can to take over his own body from madara. At this point madara will be defenseless unable to use his techniques since he will be having an inner self fight against obito. Kabuto would take advantage of this and kill madara. This is why Madara would be scared.