Doctor Janice Lester

Doctor Janice Lester was a Human scientist of the Federation in the 23rd century. 
Lester's early career included a period of service in Starfleet. She spent a year alongside James T. Kirk and they became romantically involved. The lack of opportunities for a woman to command a starship struck both Lester and Kirk as unfair, but she became embittered by the career barrier. Their relationship soured to a point where Kirk felt she was punishing him for her circumstances. Years later, Kirk said "we'd have killed each other" if they had stayed together. Lester recalled that Kirk walked out on her "when it became serious". 
Lester abandoned her career in space, but continued to study every detail of a starship's operation. She nursed powerful resentments for the femininity that stood in her way, and for the masculinity that served Kirk so well. 
By 2269, Lester was the leader of an expedition to Camus II, exploring the ancient remnants of an advanced civilization. Among the ruins, she discovered a sophisticated mechanism still in working order. The device was capable of life-energy transfer – the exchange of one being's complete personality and memory with another's. Her plans to use the device herself required several months of preparation. The expedition's incompetent physician, Dr. Arthur Coleman, bore an unrequited love for Lester. Together, they killed the expedition's entire staff by quietly exposing them to celebium radiation. The mysterious deaths announced by a distress call were meant to attract the attention of a starship, and the USS Enterprise answered the call on stardate 5928.5. 
Kirk arrived at Lester's bedside, where she appeared to be near death. Not expecting danger, Kirk was easily ensnared. Lester activated the life-energy transfer device, and was immediately thrilled by her possession of Kirk's body. The transfer would eventually return the exchanged persona to its original body, as long as both remained alive. Drugged and disoriented within Lester's body, Kirk was easy prey, but Lester's spontaneous gloating monologue prevented her from strangling Kirk before Dr. Leonard McCoy arrived. 
Lester eagerly boarded the Enterprise as the Captain, satisfied to finally have the command she felt was rightfully hers. Kirk was still a liability as long as he remained alive in sickbay, but the exclusive care of Dr. Coleman kept Kirk isolated from the crew. 
Lester's attempt to emulate a Captain's behavior was marginally successful at first, but her composure degraded at any hint of scrutiny from the officers familiar with Kirk. Dr. McCoy soon challenged the Captain's mental health, and insisted on tests that ultimately proved negative for any illness. Spock discovered the truth of the situation through a brief mind meld with the imprisoned "Janice Lester", but their attempted escape was halted by Security officers ignorant of the Captain's strange new behavior. 
"Captain" Lester convened a court martial, charging Spock with mutiny. Whatever support Lester believed she had, it all but evaporated during her farcical prosecution and hysterical closing rant. Once Lester announced the impending execution of Spock, the Enterprise crew was on the verge of actual mutiny. 
Symptoms of the waning personality transfer forced Lester to make a final attempt to kill Kirk. The two touched in a brief struggle, and the misplaced personalities returned to their proper bodies. Broken, incoherent, and sobbing in her complete failure, Janice took her accomplice Coleman's arm and wanted him to kill Kirk. Coleman offered to help rehabilitate her with the assistance of McCoy, who escorted them both to sickbay. Kirk couldn't help but feel sorry for the poor, mentally unstable, twisted-minded woman, who was driven mad not only by her ambition of craving the power to command a starship, but also her hatred and jealousy of the captain she once loved, then wanted dead. Kirk felt her life could have been as rich as any woman's, "if only..."