Defensive martial arts

[Garrion]

Hi, I'm new here but i've been trying to figure out a style of martial arts i want to try. I want something that is defensive. I know a some martial arts allow you to defend yourself against multiple opponents, but i was wondering if there is a good one for defending someone else, when you are not the initial target of the attacker.
if anybody has any ideas about a type that might match this please let me know.
Thanks
Scott