Terran
Jim Raynor
Perhaps the most important Terran leader, Raynor leads a militia/terrorist group called Raynor's Raiders, which strikes an alliance with an even more terrorizing group, the Sons of Korhal, in Episode 1 and continues to both help and fight against Terrans, Protoss, and Zerg forces through the rest of StarCraft and Broodwar.
Jim is StarCraft's ideal of a resourceful, understanding, and at times empathic hero: one who can create an army from the militia of Mar Sara and Antiga Prime; who understands what others are thinking and can work well with them; and who responds as players do to events despite his taxing experiences. He also symbolizes the slight love (in his case, for Sarah Kerrigan) that Blizzard Entertainment provides in the storyline. StarCraft is above all Raynor's story, as he somehow appears in each episode to some significant extent; it is the story of Raynor's meeting with Sarah Kerrigan, loss of her, wars against her, and possibly--that would be a logical progression--reunion with her in StarCraft 2. He is also very sensible, fighting when necessary but retreating when he must, allying with others to what he considers would do him the most good, and forming alliances with the natural partners of the Terrans, the Protoss, through much of Brood War.
Sarah Kerrigan
Sarah Kerrigan was an unwilling participant in the Confederacy's testing of Ghosts (which, according to their flawless perform ance in the game, seems highly successful, despite lore's reporting that many of the trainees died through such despicable situations as a "final exam" in which the test taker must kill a Marine holding a child hostage before the Marine fires, as told in Liberty's Crusade). After being freed by Mengsk in Uprising, she becomes his loyal servant, saying that she owes her soul to him. This becomes ironic in light of the fact that she loses her soul to the Overmind, who forces her to fight against her supposed savior.
Sarah Kerrigan is actually the smartest character in the game, although possibly tying with Duran (we have yet to see). Coupled with her control of a significant portion of the Swarms, her intellect makes her a dangerous foe. Through much of Brood War, she demonstrates her tacts and skill in diplomacy and intrigue against her foes, whom become convinced that she is their ally before realizing too late that she is only taking advantage of them.
Arcturus Mengsk
Mengsk, the leader of the Sons of Korhal, lost his father to a Confederate assassination. Ever since, he has vowed revenge against the oligarchy. After being released from prison, he has continued the firefight on Korhal, which ultimately resulted in the destruction of the planet, his home world, from a planetary barrage of nukes. It is understandable for such a man, bereft of family, to undertake extreme actions to undermine the regime. Despite this setback, however, the man has only become more determined to see the fall of the Confederacy, which does collapse following the successive Zerg infestations of the Koprulu Sector.
Arcturus' ambition is also stellar; after overcoming the Confederacy, he establishes the Terran Dominion with himself as its head, and subsequently directs the war against the Zerg. In Episode 6, Arcturus once again recovers from his lapse in power when he regains control of Tarsonis from a clandestine deal with Kerrigan, and challenges Kerrigan for the final time in The Reckoning.
Arcturus, however, is the one who has willingly sent Kerrigan to what he believed would be certain death on New Gettysburg. Unfortunately for him, she was abducted by the Overmind's minions, brought to Char, and re-hatched as Queen of the Zerg. Through much of the story, Kerrigan puts the blame of her turncoat position on Mengsk, while simultaneously defending her position as one of power and demanding of respect from her enemies.
Edmund Duke
Zerg
Queen of Blades (Kerrigan)
Most Zerg, with the exception of the Overmind and its Cerebrates, are completely devoid of personality; the one character who is not is Kerrigan, whose character traits come from her human past. Changed into the Queen of the Zerg in Episode 2, Sarah Kerrigan suddenly becomes the most powerful character in the game.
Zerg Overmind
The eternal will of the swarm and responsible for Kerrigans transformation into the Queen of Blades. This character is made by the Xel'Naga to bring order to the Zerg swarms. It controls the swarms with Cerebrates and Overlords and one time got killed by Tassadar in episode 3.
A Second Overmind was formed out of several Cerebrates in order to control the rampaging Zerg but that one was captured by the UED in Episode 5.
Kerrigan had blackmailed the Dark Templar to kill the second Overmind and that resulted in the Queen of Blades taking control of the Zerg.
Cerebrate Daggoth
Cerebrate Zasz
Protoss
Executor Tassadar
As soon as the Protoss encountered the insidious Zerg, Tassadar was dispatched, along with his Grand Fleet of the Executor, to hunt for the Zerg and destroy planets (sky-blasting--such as the instance in which the Terran battlecruiser was annihilated by a beam of energy, in the introductory cinematic to StarCraft). As the incidents on Chau Sara and then Mar Sara show, this strategy turns out to be inherently unproductive, as the Zerg act much like a cancer, each time metastasizing (spreading to other places) too early for the Protoss to react. Therefore, Tassadar abandoned his fruitless mission.
The new Queen of Blades's psionic call from Char lured Tassadar, along with Zeratul, to the planetary surface, where the two brethren unite for the first time in possibly centuries (quite a long time, considering the entire Protoss race has existed for less than a millennium). From Zeratul, Tassadar learns about the Dark Templars' energies and the Overmind's weakness.
Tassadar is at first unable to convince the Conclave--the Protoss executive, legislative, and judicial branches of government all combined in one--of the importance of his discovery. Through Episode 3, however, Tassadar manages to show the other Protoss the weaknesses in their plans and demonstrates, through slaying Cerebrates, that his plan is the only one with any hope of saving the Protoss race. Tassadar unites with Raynor in the last mission, Slaying the Beast, in which Tassadar sacrifices himself to destroy the Overmind (see the cinematic).
The Protoss Hero Tassadar is a stronger version of the standard High Templar Unit. He has double the HP as the normal unit and has a impressive 300 shield point bonus. and automatically has all the abilities readly available, with the exception of melding into an Archon. In addition to the Abilities listed above Tassadar has a Ranged Attack called Psi Assault which does 20dmg. With a short range, this attack requires you to get Tassadar in pretty close where he might come under fire. On Campaign Missions, his death will cause you to immediately fail the mission.
Praetor Fenix
Zeratul
Zeratul represents the Dark Templar and their sensibility, obtained from their encountering many situations throughout their journeys. Zeratul's Dark Templar have their secret stronghold of Shakuras, the twilight planet, and uses that sector as a staging area for his explorations, at one time leading him to a fateful meeting with Tassadar and the subsequent death of the Overmind.