Martial arts as a form of protecting someone?

People always say that martial arts is self defense to know how to get out of a bad situation, which includes evading and running away. I have asked recently about bujinkan and i was wondering about which kind of martial arts are best for protecting someone. For example if you are with your son/daughter or girlfriend who can't protect themselves or run away fast enough, what are the best martial arts for protecting others?

Would Bujinkan be effective for this? Or should I try a different martial art?