Galen Marek

Galen Marek was a Force Sensitive male born in 27 BBY. His father and mother were Jedi who had renounced the Order, and Marek lived in hiding on Kashyyyk as a child in the Clone War. Marek was the son of two Jedi Knights. Following his mother's death, Kashyyyk was invaded by the Galactic Empire as a target of the Great Jedi Purge, Kento was confronted and slain in a lightsaber duel by the Dark Lord of the Sith, who discovered the fugitive Jedi's son in the process. Gauging the orphan's raw strength in the Force, Vader abducted and ultimately raised Marek, who became Second Brother, and later Starkiller, to embrace the dark side of the Force.
In time Marek forgot his past and even his birth name, having been taken from his home as a child by the Dark Lord. Though trained in the ways of the Sith, the Rule of Two dictated that there could only be two Sith at a time, and therefore Marek could not consider himself a true Sith while his master was still apprenticed to Darth Sidious. Therefore, Vader added Marek to the ranks of the Inquisitorious, giving him the rank of Second Brother. Marek trained and served the Sith. In 10 BBY, Second Brother sensed a tremor in the Force, and went to a Mandalorian moon, where he confronted Cal Kestis. He beat Kestis to the near point of death, before the Jedi managed to slightly get through to him, having a memory of his resurface. Kestis later escaped along with his companions Merrin, Cere, BD1 and Clan Skirata. 
In 9 BBY, Second Brother was sent to Felucia, where he found former Jedi Master Shaak Ti in hiding. He managed to kill her, a beat which impressed Vader. Vader, in secret, gave him the codename Starkiller. He said that Starkiller was to be his secret apprentice while still under the guise of being Second Brother. Starkiller accepted, and was given the training droid PROXY and pilot Juno Eclipse as companions. Starkiller was sent out on several more missions over the years, killing many more Jedi, and blinding Rahm Kota, though he escaped.
The secret apprentice completed his missions, prevailing against his more experienced targets, only to be betrayed by his master after the Emperor confronted them. Stunned by this turn of events, Starkiller nevertheless remained loyal to Vader, and began fomenting a rebellion against the reign of the Galactic Empire as part of his master's plan to depose Sidious. The mission was another ruse, however. After the apprentice convinced Garm Bel Ibis and Riyo Chuchi to join him, Vader had the senators arrested and left Marek for dead, betraying him once more, revealing he was but a tool in a plot by Sidious and Vader to crush the rebellion before it even began, however he lived. 
Resolving to rescue his allies, he first saved Eclipse and PROXY, before seeking out Kota for help with the rescue mission. Though Kota was reluctant, he agreed, and taught Marek how to see the Force through a light side view, allowing Marek to cleanse his lightsaber crystals. Marek managed to then infiltrate the Executor before the Emperor could execute his Senator allies, and also confessed his mutual love for Juno Eclipse. Although he was killed in the ensuing battle aboard the battlestation between himself against Vader and Sidious, Eclipse, PROXY, Kota, and the senators escaped, and would continue Marek’s rebel cell, calling it the ‘Marek Cell’ in his honor.