Empress Leia!
Abbreviated backstory:
Leia’s ship wasn’t boarded by Vader.  She got to Obi-Wan herself; he trained her (briefly).  She ended up as the New Hope.  Things proceeded roughly similarly to the movies except with Leia’s and Luke’s roles swapped, until the end of Return of the Jedi.  Unlike Luke, she agrees to turn to the Dark Side.
Sidious immediately tries to get her to kill Vader, as he now has a new, fresh apprentice.  She refuses and uses the Dark Side to attack Sidious.  Vader is now at the point where he has to choose between his family, who has just gone against her basic morality to side with him, and the master who has just ordered him killed.  The choice is more difficult than you’d think, but he sides with Leia.
They battle Sidious together.  Sidious electrocutes Leia, causing a small Lichtenberg figure scar on her face as her blood boils beneath her skin.  She also loses three fingers on her right hand during the battle.  After Sidious’s defeat, Leia rules the Empire with Vader as her iron fist.
