Serene

"Why have you killed my Master, Serene?" "Because I love you, Master..."-Serene and Darth Vader

Serene was a Dark Jedi and prospective apprentice of who secretly served the Empire and was known  only by Vader and fellow apprentice. Vader trained her from the age of two after her entire family was killed in the aftermath of the. Serene was raised by Vader, her ownly companion being Marek, whom she felt feelings for but was rejected by because Marek felt he had to compete against her as Vader's apprentice. Much like Marek, Vader trained Serene to be a ruthless assassin and a powerful Force-sensitive, not as powerful as Marek, but just as skilled and less impressionable. Instead of hunting down and killing Force-sensitives like Galen, Serene's acted as a tracker for Force-sensitives because of her gift in, finding Sensitives to be trained as Inquisitors and recuriting them, peacefully or forcefully. Palpatine never caught wind of Vader having a second apprentice, for she covered her tracks more adeptly than Marek did. Her relationship with Marek would go from a love interest to a full scale rivalry, with both of them competing for Vader's admiration or possibly, affection.

After Mareks death, Vader began to treat her much more gentler than he had initially. After seeing how expendable he was in Sidious' eyes, Vader had grown even more disheartened to his Master. However, he also grew more distraught to the point of physical pain, stemming from the fact that Sidious' truly didn't care for him and that he was only a tool. Serene suffered from her master's suffering and resolved that, if she was to free her Master of this everyday pain, she was to kill Sidious.