Brother's Keeper was a neutral side mission in Infamous. Cole was asked to free a loved one from a group of Dust Men. Down near the docks, MacGrath was contacted by a medic whose brother has been captured by the Dust Men. He was forced to dress up in their uniforms and patrol with them across the Warren. The medic then gave Cole a picture of his brother wearing the Dust Men clothing, who was the only one of them who had a paper bag for a mask. Upon finding the group of Dust Men, Cole took down each one, except the medic's brother. Once it was clear, he helped the captured man to his feet, and the latter reunited with his brother.
Walkthrough tip: Using Precision can be useful for this task, as it can also help pinpoint the captured brother, aside from using the picture. Also note that the brother will take cover during the fight, and will not attempt to attack you.