Alder was responsible for patrolling the base's outer perimeter under Derlin's command, and he was the officer to inform Derlin that fellow Rebels Luke Skywalker and Han Solo still had not reported in to the base after Skywalker had gone missing during a routine scouting mission. Despite their absence, Derlin tasked Alder with closing the base's shield doors for the night, reluctantly trapping both missing men out in the cold.
By the time Alder eventually received assignment to the Alliance High Command headquarters of Echo Base on the remote ice planet Hoth, he had already served for years with MajorBren Derlin in the Alliance Army, fighting in some of the Rebel Alliance's most perilous campaigns of the Galactic Civil War, including the Battle of Nentan,[5] an effort to evacuate refugees from Imperial forces.[11] Alder's expertise in terrain scouting and native creatures impressed Derlin, and his abilities made him among the first to arrive at Echo Base. As one of the finest forward scouts present,[5] Alder's duties included patrolling the facility's outer perimeter on the planet's icy wastes,[4] under Derlin's command.[1]
When base personnel Luke Skywalker and Han Solo went missing out in one of Hoth's brutally cold nighttime blizzards during a routine scouting expedition in early 3 ABY, Alder coordinated a search party for the two men.[5] Although Echo Base waited as long as possible before closing its protective shield doors for the night, in hopes that the two would return safely, Alder's responsibilities required him to report to Derlin that with Hoth's dropping temperatures threatening the conditions of the base's interior, Skywalker and Solo had still not reported in. The situation forced Derlin to inform base leader PrincessLeia Organa that the doors would have to be closed for the night, to which she reluctantly agreed. With her approval, Alder carried out Derlin's order to close the base's shield doors, effectively trapping Skywalker and Solo out, leaving them to their fates.[3][6]
Alder later fought during the subsequent Battle of Hoth, the large-scale Imperial invasion of Echo Base, serving along the main Rebel defensive perimeter, where he helped to organize snowspeeder groups against the attacking line of Imperial assault walkers. Despite the heavy losses suffered by the Rebellion during the battle,[5] which proved to be the Alliance's worst battlefield defeat of the war,[12] Alder survived the engagement.[5]
Cal Alder was highly trained as an expert scout in his service to the Rebellion,[7] among the best stationed at Echo Base. His expertise in terrain and native creatures made him one of the earliest to arrive at the newly completed outpost.[5] When Alder informed Major Bren Derlin that Luke Skywalker and Han Solo still had not checked in from Hoth's exterior, he did so with crisp efficiency, initially unaware that Princess Leia Organa, whose thoughts rested heavily on her beleaguered friends, was nearby and could overhear his report. Derlin calmly raised a hand to cut him off mid-sentence in order to lower Alder's voice for the Princess's benefit.[3]
Alder's Hoth equipment included a DH-17 blaster pistol and protective goggles.[15] He was also trained in riding a tauntaun,[2] one of Hoth's native beasts that the Rebels tamed for use as a mode of transportation.[11] Alder had bluisheyes and light skin.[1]
While the Who's Who in Echo Base article identifies Alder as being a lieutenant in his respective character entry, the "CorporalCorman Quien" entry within the same article lists Alder as a sergeant.[5]