Post a character who has a dark side, dark past, dark secret, or all of the above

Itachi Uchiha (Naruto Shippuden)

well every one knows about his dark past..what all he went through.what all he sacrificed........i dont have to tell u guyz...bcz u already know.........he he he heh..

Lucy/Nyu from Elfen Lied and Yuno from Nikki Mirai

Ciel Phantomhive from Black Butler/Kuroshitsuji
He had a dark past
SPOILER
(If you haven't seen Black Butler/Kuroshitsuji don't read ahead)

Ciel's parents died in a fire on his birthday
Since that day he was trying to find out who killed his parents
He met a demon, Sebastian Michaelis, who is now his butler
They search to find clues, or facts about about his parents murder

There are LOTS I could list...

But, since the last few Naruto Shippuden episodes have been about his past, I shall post Kakashi Hatake.

Mondo Oowada from Dangan Ronpa killed his own brother.

Latooni Subota had a Dark Past.
When she was created ( because she was a robot ), her adjustments are all uncomplete. Being like that, she was tested, and they all discarded her on The School. She cried, and her big sister, Ouka, says, " Don't worry, Lat. Don't cry anymore. I'll be there by your side." Latooni asked her where she was going. Ouka told her that she will going to be tested again and will come back for a long time. And, after 8 years, Ouka doesn't came back anymore. Until she was hired by the EFA, Ouka hasn't returned yet. Lat tried to forgot the memories, and she finally did it. And when it happened, Ouka returned, hypnotised. Ouka tried to kill Lat, and Lat is almost killed. Good thing, Princess Shine was there, fighting for his father too, Shine believes that the enemy is the one who killed his father. After 6 months, Lat encountered Ouka again. Ouka was badly hypnotised and was exploding already. The time has come when Ouka finally get back her memories, and saved Lat. After 2 years, Lat wouldn't forgot about her sister's sacrifice.