The Alliance to Restore the Republic was formed to combat and usurp Emperor Palpatine’s Galactic Empire. The Corellian Treaty stated:
“We agree that the time for diplomacy and politics has passed. It is now time for action… Therefore let this be an official declaration of rebellion! Today, we all vow to change the galaxy, and one day the galaxy will indeed be free! " — Bail Organa
Signatories included Mon Mothma, Bail Organa (and his daughter Leia), Garm Bel Iblis, former Jedi Master Rahm Kota, and the man whose family crest would go on to be a symbol for the Alliance, Galen Marek (a.k.a. Starkiller).
Marek was apprenticed to Darth Vader in the early years of the Great Jedi Purge, and was sent to assassinate Rahm Kota. He failed, but blinded the Jedi Master, and severed him from the Force. Later, after hunting more survivors of Order 66, Vader and Palpatine schemed to use Starkiller as a rallying point for rebellious senators and politicians plotting against the Empire. Traveling back to his former home on Kashyyyk, he found a blue lightsaber crystal that belonged to his father, and had a vision through the Force that revealed to him the truth of his origins — he was he son of two Jedi, and Vader killed his father.
He left the hut a changed man, determined to end the rule of the Emporer. When the senators who led the Rebellion were captured and taken to the Death Star, he came to their rescue. After ensuring all the senators made it safely away, he was killed by the Emperor.
“He was meant to root out the Rebels, not give them hope. His sacrifice will only inspire them…” — Emperor Palpatine.
The Rebel Alliance chose to honor Marek’s sacrifice by donning his family’s crest as the symbol of their hope.