Galen was Jean-Luc Picard's archaeology instructor at Starfleet Academy; he saw great potential in Picard and was very close to him, but ultimately Picard refused Galen's offer of becoming an archaeologist, which heavily disappointed Galen. Until 2369, the two hadn't met for thirty years, but Picard kept track of the professor's work and Galen of Picard's. Just before Galen's death, Picard stated that he had been the father who had understood him, in stark contrast to his real father. Galen, himself, had children of his own, however, none chose to follow in his footsteps, which reinforced his close relationship with Picard.
In 2369, Galen was close to deciphering the code and paid a surprise visit to Picard on the USS Enterprise-D, collaborating with Commander William T. Riker to surprise Picard with the gift of a Kurlan artifact he had recently discovered, the Kurlan naiskos. Although surprised to see his old teacher, Picard assured him that no one could be more welcome aboard the Enterprise-D, and was particularly touched at the gift given the rarity of the piece. However, when Galen asked Picard to come with him for the rest of his journey, Picard again disappointed Galen by refusing in favor of his responsibilities on the Enterprise, and Galen left the ship immediately, off to rendezvous with a Vulcan transport destined for Deep Space 4.
Shortly thereafter, Galen died as a result of injuries suffered when his shuttle was attacked by the Yridians working for the Cardassians. Before he died, Galen admitted to Picard that he had been too harsh with him. Picard was later able to complete the late-Galen's discovery. (TNG: "The Chase")
Galen was played by actor Norman Lloyd.
His mirror universe counterpart (β) was mentioned in the novella "The Worst of Both Worlds". He was a self-educated Terran archaeologist and the mentor of Luc Picard. Many years prior to 2371, he was executed by the Klingon-Cardassian Alliance because of his "seditious" theory that Terrans shared a common ancestry with Klingons and Cardassians.