Darakian is a rogue protoss Dark Templar. A solitary, silent individual, Darakian is like many Dark Templar in that he has never abandoned his love for Aiur. He considers the Khala a genuine threat to the protoss.
At some point, likely during the zerg invasion of the Koprulu Sector, Darakian abandoned the Dark Templar space vessel on which he lived and offered his services to the Alliance, seeking a way to eradicate the zerg before they destroy protoss civilization. While he has no great love for terrans, he's willing to work with them to eradicate the zerg. In turn, the Alliance only tolerates him for his skills and knowledge of the zerg.
In addition to the standard abilities of the Dark Templar, Darakian has the ability to cast psionic bolts and psionic storms. His ability to cast the latter would suggest that Darakian has an extraordinary amount of willpower, given that he cannot use the Khala to control his abilities.