Elbow is an enemy Navi, they have the negate all damage divisible by 3 ability. However, when they join your team, it becomes a "good" Navi, which has the ability to give extra directions when navigating. Another example is Cap, enemy Pirate, who has a chance to intimidate you. However, your good Pirate grants you free deals from pirates. These and a few others(like Dandy, Mate, Boy) crew members' ability changes when they change sides.