Ka'hairal Balak is the main antagonist of the Bring Down the Sky DLC in Mass Effect and the possible main antagonist of the side quest mission "Batarian Codes" in Mass Effect 3. He is the captain of a batarian terrorist group that raids Asteroid X57 in 2183. Unlike the other batarians, who only planned to make a quick slave grab, Balak is on a mission to crash Asteroid X57 into the human colony of Terra Nova. Balak hates humans because they claimed territory and resources that he believes should have belonged to the batarians during the early stages of humanity's major expansion into the Skyllian Verge. He sees his actions as reprisal for the Skyllian Blitz on Elysium.
Balak rigged the fusion torches propelling the asteroid to execute burns that put it in a collision course with Terra Nova. After Shepard lands in the Mako and begins on shutting down the fusion torches, Balak sends his forces after the Commander. Balak then hunts for the hiding engineers in the main facility who were aiding Shepard. When he finds Kate Bowman, he demands to know who is attacking his people. When Kate refuses to answer, Balak kills her brother and sends Charn to find Shepard.
When the Commander launches an assault on the main facility, Shepard confronts Balak, who has planted a bomb in the offices where the hostages are being held. Balak talks angrily about the thwarted Skyllian Blitz (if Shepard is a War Hero, Balak holds Shepard personally responsible for the Blitz's failure) and about Torfan (if Shepard is Ruthless, Balak accuses Shepard of murdering batarians).
Shepard has a choice to make, to either attack Balak to ensure his death, or release him in order to save the hostages, including Kate Bowman.
Balak detonates the bomb if Shepard chooses to attack him, killing the hostages, though he soon falls under the Commander's assault. Cornered, he taunts Shepard about the similarity of their actions, rhetorically asking the Commander "who's the real terrorist" now. Shepard can first fire a warning shot, graze him, then seriously wound him, or alternatively just verbally spar with him, but that doesn't stop Balak from spouting batarian jingoism, unless Shepard kills him outright.
Judging by Balak's reaction when Shepard asks him who is in charge of the batarians' terrorist actions, it is likely that Balak, as well as other batarian terrorist groups, answer to a higher authority. Balak's fate hinges on Shepard's decisions: he's executed after Shepard learns everything there is to learn from him, he's left to die there and then, or he's dropped off on Terra Nova to be handed over to the Alliance.
Mass Effect 2
If Shepard let go of Balak in Mass Effect, a Citadel news feed mentions that Balak is still at large.
Mass Effect 3
During the Reaper War, when Shepard investigates a usage of illegal batarian codes on the Citadel, Balak appears and holds Shepard at gunpoint (If he was left alive in Mass Effect). Balak explains that his government twenty years ago discovered the Leviathan of Dis (an actual Reaper corpse) and the best batarian scientists were hired to study the Reaper corpse in order to gain the technology to advance the batarian race and rise to glory. Unfortunately the scientists got indoctrinated.
When the Reapers invaded the galaxy and started their invasion by attacking Khar'shan, the batarian homeworld, the scientists that were indoctrinated betrayed the batarians by disabling planetary defenses and causing batarian allied ships to fire on one another. As a result, the Reapers easily invaded the batarian homeworld, leading to the fall of the batarian homeworld and it's government.
Balak managed to escape to the Citadel. With little to do since his homeworld and his government had fallen, Balak decided to aid the remnants of the batarian fleet against the Reapers by hacking Council transmissions. Balak claims he is the highest ranking military officer left in the Hegemony. Thus, he holds Shepard responsible for the destruction of his culture and wants to kill the Commander for purely vindictive reasons, most notably the destruction of the Alpha Relay that stopped the Reapers from an early invasion, but costed the destruction of the Bahak system, as well as three-hundred thousand batarian lives lost.
Shepard can choose to kill Balak or convince him to lend him support in the war against the Reapers, of which he'll reluctantly agree to lend his fleet for the war.