As detailed in "The Last Jedi" companion novel "Leia, Princess of Alderaan" by Claudia Gray, Leia Organa and Amilyn Holdo were sent to the Apprentice Legislature in the same period, nearly four decades before the events of "The Last Jedi," as representatives of Alderaan and Gatalenta, respectively. At the Legislature, Leia and Holdo enter the same pathfinding class, where they learn the art of braving and surviving challenging natural environments.
Although many of the Legislature's students, including Leia, are initially somewhat put off by Holdo's flamboyant style and eccentric demeanor, the Gatalentan eventually manages to impress and enrage her peers in equal measure with her impassioned, fearless opposition to Imperial tyranny on the Legislature's floor. The anti-imperial like-mindedness between Holdo and Leia, coupled with their similarly outspoken and rebellious personalities, lead them to become close friends. Leia, who is involved with the growing resistance movement against the Empire, struggles for some time with whether to reveal its existence to Holdo and invite her in — but she grows to trust Holdo enough to do so.
At that point, the two became lifelong partners in revolution, eventually leaving Imperial politics together to officially board the Rebel Alliance, where they mutually supported each other's ascension through the political ranks. Then, years after the Alliance's victory, when Leia forms the private military force known as the Resistance to combat the First Order, Holdo is among the first rebel peers from the old days whom Leia enlists to establish a trustworthy chain of command.
