Ackbar was born shortly after the Clone Wars on the watery world of Mon Calamari. He was brought up with his 8 siblings in a stern religious household. Ackbar was raised to mistrust, but tolerate the squid-like Quarren, with whom the Mon Calamari shared the planet. In a stunning move for the Mon Calamari, who, unlike the Quarren, had remained loyal to the Emperor during the Clone Wars, the Empire went against the wishes of the Mon Calamari and opened the planet for commercial fishing. The primary beneficiary of this policy seemed to be Admiral Motti's seafood business. While out for a swim with some of his brothers, Ackbar was caught in a commercial lobster trap. While his parents were able to buy his freedom, the experience left Ackbar deep emotional scars and a fear of being trapped.
The Empire eventually had enough of the Mon Calamari's complaints and decided to enslave the entire population of the planet. Ackbar was assigned to a ship-building unit and showed a natural ability in that field. He was discovered by Grand Moff Tarkin, and taken as his personal slave. Ackbar learned a great deal about military strategy from Tarkin and the two developed a mutual respect for one another. Had it not been for the Empire's strict "humans only" policy, Ackbar likely would have become an Imperial officer. As the first Death Star neared completion, Tarkin became its overseer. He chose not to take Ackbar along due to his (correct) assessment that Admiral Motti would try to eat him. Instead Ackbar was sent to Coruscant to work in the Imperial IT department, where he was spared from the destruction of the Death Star. During his time in IT, Ackbar gained access to many classified Imperial documents and began to see the oppression that many were feeling as a result of Imperial policies. He made the decision to defect to the Rebel Alliance, but his fear of being trapped caused him much trepidation. He developed a careful plan to escape Coruscant and travel to Corellia, which he learned was a hotbed of Rebel activity. While participating in a service call to the Empire's e-mail solicitiation office, Ackbar excused himself to use the restroom, put on an elaborate disguise, and boarded a transport to Corellia. The rest of the IT department did not notice his absence for 7 weeks.
Once on Corellia, Ackbar began asking strangers if they knew how to join the Rebellion. One of them suggested that he go ask a group of Imperial Stormtroopers, but, sensing a trap, Ackbar declined. Eventually, he met a pilot named Wedge who took him to Mon Mothma. Based on his background working with Tarkin and his knowledge of Imperial practices, he was immediately commissioned as an Admiral. One of his first moves was to hire a staff of Mon Calamaris and other freaky aliens. After a string of successful strikes, Ackbar rose to the position of top military commander. He worked closely with Mon Mothma to develop overall strategy. When some Bothan spies stumbled upon some details about a new Death Star, Ackbar saw the opportunity to defeat the Empire and end the war. His plan required the entire Rebel fleet to supply coverage so a few smaller ships could fly into the incomplete Death Star and destroy it while the Emperor was onboard. The key to the mission was a small group of Rebel commandos knocking out a shield generator on the forest moon of Endor. Ackbar was confident that the plan would catch the Imperials by surprise.
Battle of Endor
When the Admiral and his fleet arrived at the location of the battle station, he was alarmed to learn that the shield had not yet been disabled. He was further horrified by the fact that the Death Star's main weapon was fully operational, and was being used to pick off capital ships. His mind raced back to his youth and he felt the walls of the lobster trap around him. Realizing that he had once again been trapped, he ordered a retreat, but Lando Calrissian took charge like only Lando could, and talked him out of it. Ackbar was able to compose himself and organize the rest of the battle. He was able to buy enough time for the ground forces to destroy the shield generator, and for the fighters to enter the Death Star and destroy it.
Ackbar remained one of the leaders of the New Republic for many years. He eventually retired or died or something.
Ackbar designed the B-Wing, and named it even though it didn't really look like any letters. Several years after the Battle of Endor, where his fighter shined so very brightly, Ackbar finally hit upon the idea that it came from "battle-wing". It annoyed him until his last day that he couldn't have thought of that a lot sooner.