Captain Barca, Pirate Clans
Barca was one of pirates did not agree with Jonas Simbacca's decision to come into the war on the side of the Exofleet. This opinion was based not only on past hatred for the ExoFleet, but also the fact that if the Pirates had joined the Neosapien cause, their victory would have been assured, while in allying themselves with the ExoFleet victory was not a foregone conclusion. Therefore, Barca struck a bargain with Phaeton. For Barca's covert assistance the Pirates would receive Venus (which was preferable to Mars, the planet promised to the Pirates by Winfield) at the end of the war if the Neosapien Commonwealth emerged triumphant.
Although Phaeton had no intention of fulfilling his side of the bargain, Barca did much to sabotage ExoFleet. On various occasions he: killed fellow Pirate and formidable warrior Jubail, convinced Hallas to assassinate Simbacca (which failed), constantly provided information to the Neosapiens, planted a bomb that crippled the Resolute II, led ExoFleet forces into traps, and conspired with Typhonus to get a ship loaded with explosives close enough to the Resolute II to destroy it (during which he himself was killed). The death of Barca and the crew that followed him in "Fire Ship" is a severe loss to Phaeton, as there are no more acts of sabotage seen being performed to obstruct the ExoFleet from then on.
Originally, Barca seemed content to control events from behind the scenes, trying to manipulate Jubail and Hallas into acting to achieve his ends. As time progressed, however, he began to aspire to leadership, and hoped to claim it once the war ended victoriously for the Neosapiens. We can assume that Barca did earn his rank as Captain of a Pirate destroyer in a manner considered appropriate by a Pirate because many of the Pirates that we see are openly on his side. His crew on the Avenger share his opinions regarding the ExoFleet as those seen at Port Chaos or are his friends anyway such as Jubail and Hallas.