|Position|| Lieutenant junior grade
Richard Castillo was a Starfleet lieutenant junior grade serving aboard the USS Enterprise-C in 2344.
In that year, the Enterprise-C, under the command of Captain Rachel Garrett , responded to a distress call from the Klingon outpost at Narendra III, which was under attack by four Romulan Warbirds. As the Enterprise-C was defending the outpost, the weapons discharges resulted in the creation of a temporal rift, through which the badly damaged Enterprise-C drifted, arriving in the year 2366. Upon emerging from the rift, the crew encountered their ship's immediate successor, the USS Enterprise-D, whose captain, Jean-Luc Picard , informed them that the Federation was at war with the Klingon Empire, and was on the brink of defeat.
As the crew of the Enterprise-D assisted their counterparts in making repairs, Castillo made the acquaintance of Tasha Yar, the Enterprise-D's chief tactical officer. It was eventually determined that the Enterprise-C's journey through the rift had resulted in the creation of an alternate timeline, and that, if they wished to restore the proper flow if history, the Enterprise-C would have to return through the rift, in the middle of the battle with the Romulans. Although much of the crew was initially skeptical, as returning to the battle with the Romulans would almost certainly result in their deaths, Captain Picard pointed out that, if a Federation starship were to sacrifice itself in defense of a Klingon outpost, such an act would be seen as extremely honorable by the Empire, and would perhaps avert decades of war. Before the Enterprise-C could depart, however, both ships came under attack by a Klingon Bird-of-Prey, resulting in Garrett's death.
As the highest-ranking crewmember left alive, Castillo took command of the Enterprise-C, and prepared to take the ship back through the rift. This caused a certain amount of distress for both Castillo and Yar, who had developed romantic feelings for one another. Eventually, however, Yar decided to join Castillo and the crew of the Enterprise-C when she learned that she had died a meaningless death in the other timeline. Eventually, Castillo led the Enterprise-C back through the rift, repairing the timeline and leaving the crew of the Enterprise-D with no memory of the events which had transpired.