Thane was created for the video-game Mass Effect 2, and made his first comic book appearance in Mass Effect: Foundation #8, written by Mac Walters with art by Jeremy Barlow.
Eight centuries before the events of Mass Effect, the drell homeworld Rakhana began a steady decline into an environmental apocalypse, triggered by burgeoning overpopulation and uncontrollable industrial expansion. Internal conflicts about the division of dwindling resources raged across Rakhana, until the hanar discovered the planet in the 20th century, and proposed to try and save as many of the drell as they could from their rapidly dying world. Ultimately, the hanar managed to evacuate close to 375,000 drell from Rakhana, and were forced to leave the rest to their fate.
The drell who were rescued from Rakhana were relocated to the hanar homeworld Kahje, where they assimilated into hanar society first as respected guests, and then as citizens and brethren of the Illuminated Primacy. The drell were grateful to the hanar for their salvation, and earned their keep as defenders and soldiers of the hanar government. The hanar could move quickly in aquatic environments, and produced natural toxins they could discharge through skin contact, but they were physically weaker than most of the other Citadel races and could not handle firearms well. To compensate for this, they forged a treaty with the drell known as the Compact, wherein a substantial portion of Kahje's armed forces would be comprised of drell who were expertly trained from a young age. Any drell selected under the Compact was free to decline, but it was considered a honor to be chosen, and thus few rejected it.
Born in 2146, Thane was one such drell, and at the age of six, his family had volunteered him for training under the Compact. He was taught how to kill with the expertise of a true assassin, and before his 13th birthday, he had already successfully executed his first target for the Primacy. After learning of Thane's past, Commander Shepard voices outrage over the fact that the hanar would mold a child into a killing machine, but Thane quickly clarifies that they were very reluctant, only doing so because the circumstances mandated it --- stating that the hanar "regretted their need" for his talents.
Like all drell, Thane possesses an eidetic memory, and can remember any event that he has ever experienced or seen down to the smallest detail, including specific colors, scents and sounds. In the midst of conversations, he is prone to becoming "lost" in his memories, drifting into quick soliloquies of vivid moments that spring to mind. During one of his assassinations, he was stopped from killing his target when a drell woman named Irikah stepped into his line of fire, shielding the target with her own body and refusing to let him finish his kill. Startled, Thane retreated from the scene, finding a different time and place to complete the job. But the memory of Irikah stuck with him, returning to his mind again and again. She reminded him of Arashu, the drell goddess of motherhood and protection, and he felt compelled to seek her out and beg her forgiveness. Eventually, the two fell in love and were married.
The hanar released Thane from the terms of the Compact so that he could start a family and lead a normal life, but Thane soon found that the only true skillset he possessed was that of an assassin, and so he freelanced as a mercenary to pay the bills. Irikah bore him a son, Kolyat, but Thane was an absent father, always off on a mission while his son grew up without him. Years later, Thane assassinated a batarian merc commander, and his vengeful soldiers purchased information about their leader's killer from the Shadow Broker. While Thane was away, they broke into his house and murdered Irikah. When Thane returned, he buried his wife and left Kolyat in the care of relatives, before hunting down Irikah's killers and killing them slowly and painfully, a contrast to his usual swift and efficient methods. These were the only targets he had personally earmarked for death, and the only time he would let emotion drive his actions.