Each friend (and three of the girlfriends) provide special unlockable abilities in return for maintaining good relationships with them.
There are five friends in total, which become available at different times during the storyline.
Each friend offers a 'special ability' in return for maintaining a good friendship.
In order to unlock a friend's special ability, you must have a high 'like stat' for that friend, and this stat is based on how regularly you hang out with your friend.
Jacob becomes a trusted friend of Niko's as the story progresses.