Rocket is a raccoon, or raccoon-like creature, enhanced by illegal genetic experiments into a sentient and technically proficient being. This experience was apparently quite traumatic as he remembers having been "torn apart and put back together." At some point in his life he met the sentient plant-life Groot, with whom he became partners in bounty hunting.
Rocket spent most of his life as a bounty hunter, teamed up with Groot to perform miscellaneous (and typically illegal) jobs, such as kidnapping and theft, in search of income. He ran into frequent trouble with the law and was imprisoned on numerous occasions, but managed to escape from confinement each time. Despite being a very capable bounty hunter he was frequently taken advantage of by both his employers and competitors alike, at times reaching the verge of bankruptcy.
His search for employment eventually brought Rocket to Xandar, where he staked out potential bounty targets in a crowded plaza. Eventually, he spotted Peter Quill - wanted for 40,000 Units by Yondu - and attempted to capture him during a fight between Quill and Gamora. The fight led to all four being arrested by the Nova Corps.
The group was later taken to the Kyln to be imprisoned, though Rocket was unfazed, boasting openly of his prison-escape history while being led inside. He called out Gamora for being a "lackey of a genocidal maniac" and daughter of Thanos, though she claimed that she was planning to betray Thanos and Ronan and sell the orb to a third party. Rocket himself was called a raccoon by Quill, though Rocket shrugged this off by claiming he was one of a kind.
Rocket and Groot quickly established themselves as hazards to the prisoners when one of them attempted to harass Quill, whom Rocket declared was their "booty". He was less concerned with Gamora's fate, and attempted to discourage Quill from intervening when Gamora was attacked by several other prisoners (including Drax), but Quill ignored him and successfully talked the others out of killing her. When Gamora later confessed that the Orb was priced by her buyer for 4 billion units, and offered to split the profit, Rocket agreed to formulate a plan to break the four out of the Kyln.
The next day, Rocket briefed Quill, Gamora, and Groot on his plan during lunch. He requested that Quill retrieve a prisoner's prosthetic leg, and had Gamora tasked with acquiring one of the guards' control devices needed to access the security watchtower. Rocket then explained that the final piece for their freedom is a control panel holding a Quardrix battery, which if removed would trigger the prison's emergency lockdown. However, as he was explaining this, Groot had wandered to the control panel and ripped the battery off, throwing Rocket's plan unexpectedly into action. As Quill and Gamora retrieved the items needed, Rocket berated Groot for causing chaos. Security drones and guards arrived and fired upon Rocket and Groot. However, Drax came to their aid and tossed Rocket one of the guards' weapons, much to the bounty hunter's pleasure.
The items required for the breakout were retrieved and the group, including Drax, locked themselves within the security watchtower. When Quill presented the prosthetic leg, Rocket laughed him off, revealing that the request was a morbid joke and he didn't need it at all. Rocket then initiated his plan; he switched off the prison's artificial gravity, took control of the security drones and then used them to propel the watchtower room out of the cellblock and into the prison's docking bay. Rocket and the others boarded Quill's ship the Milano to make their escape, but were forced to delay their departure when Quill went back to recover his Walkman.