Missy, from Doctor Who. She and the Doctor grew up together as best friends and then wound up being the last two people of their race (well, the others are still out there, but they're all sort of... stuck). Missy sort of fell off the tracks while the Doctor is just this guy who tries to do good. But he eventually sways Missy back to the side of good, but what's really tragic is that she ends up dying... without the Doctor even realizing that she did turn good. She's just so nuanced and funny. And Michelle Gomez plays her perfectly.
Also, I love that bit that goes...
MISSY: Well, I am that mysterious adventurer in all of time and space, known only as Doctor Who. And these are my disposables, Exposition and Comic Relief.
NARDOLE: We're not functions.
MISSY: Darling, those were genders.
This was a triumph... I'm making a note here: huge success...