Darryl Adin was a Starfleet security officer who was part of a team who rescued Tasha Yar from the failed New Paris colony in the 2350s.
Adin took Tasha under his wing and was proud when she entered Starfleet Academy. He came back in her life after she graduated and the two began a romantic relationship.
On a mission of trade aboard a ship full of junior officers, an Orion ship attacked and boarded the ship, killing several officers and stealing dilithium. With Adin's leadership, the ship made it to a starbase. But upon arrival, Adin was immediately arrested for aiding the Orions in the attack. Charged with treason and murder, he faced a trial where he was found guilty, but escaped custody.
Years later in 2364, while Tasha was serving aboard the USS Enterprise-D, she and Data went on a mission to the planet Treva, a world that has asked for Federation assistance. Kidnapped by rebels, Tasha learns that the world's leader has been abusing her power. Also, she finds Darryl Adin, now aiding the rebels in their fight, going by the name the Silver Paladin.
After working with him again, she struggles whether to let him go or turn him in, but eventually she decides that she must bring him back into custody. On board the Enterprise, Data researches the records of the Orion attack many years ago, and Adin is proved to be innocent. He is freed, but Tasha decides to stay on board the Enterprise. (TNG novel: Survivors)
Adin is still leading the Silver Paladin several years later. (TNG novel: Metamorphosis)