Post a character who has a dual/double-sided personality

Hatsuharu Sohma is sweet and calm and then he goes crazy if you piss him off

Lucy/Kaede from Elfen Lied. When she gets shot in the head, she loses her memory and then develops a second personality; Nyu. :D

Echo and Zwei (also called Noise), from Pandora Hearts

Allelujah Haptisum - Gundam 00

Once again, Kagura Sohma! <3 She is usually really cute and kind, but when she is around Kyo... and she is pissed of... let's say she can't control her emotions ;DDD

Sora from Sukisho has another personality called Yoru

Takashi Kawamura from Prince of Tennis is a soft-spoken person off the court, very gentle and shy, though still outspoken and stubborn when he takes a decision. He is very caring and does tend to worry a lot (regarding his future and friends).However despite his quiet personality, he becomes extremely aggressive and loud once he grabs his racket, or any racket. This change dramatic change is marked by him shouting "Burning!!!" "Great!!!" "Baby!" His personality along with these keywords when he holds a racket is most likely just for comic relief, but his personality change does have a large impact on his skills. While in his Burning Mode, he has a tendency to speak in English and to call people, mainly underclassmen, monkeys. Once in this persona Kawamura becomes filled with confidence and does not think before he speaks. He also sometimes refers to himself as "Ore-sama" (My magnificent self, The great I) during this phase.

Jugo (Naruto Shippuden)

he have dual personality......he have a friendly & kind personality.........& then he have another personality with high killing impulse............he he he ehe

Shiro (Deadman Wonderland) has a dual-sided personality : the first one is kind and very innocent while the other personality is bloodthirsty and merciless.

Natsuki from Uta no prince sama

Ukyo from Amnesia

Moka Akashiya

Would Bakura-kun from Yu-Gi-Oh! count? There's Ryou Bakura (left) and Dark Bakura (right) Although it seems Dark Bakura,could take him over at anytime and he could even act like him.

Yukiko from 11eyes!

Her personality changes when she removes her glasses.

Akito and Agito from Air Gear. He can go from a soft and cute little kid to a kick ass boss in two seconds flat! To change his personalities, all he has to do is move his eye patch from one eye to the other!

Teito Klein and his dual personality, the eye of Mikhail

Launch from Dragon Ball. She's originally kind and blue-haired, but when he sneezes, she turns blonde, gaining a rough, trigger-happy demeanor.

Grimmer from Monster

zetsu

All the main characters from Persona 4 The Animation. The whole show is about having a different persona, the true person behind 'the mask.'

I'll go with Kanji Tatsumi because he is the most dramatic. Delinquent tough guy who likes cute things and knitting. Yeah

I would say Sebastian from Black Butler.

The side that is a perfect gentlemanly butler and the side that is the perfect demon.

Yuno gasai from mirai nikki, she can go from being a cute and normal girl to a psycho stalker murderous babe. She can be your favorite or last favorite character at any given time, (she's awesome all the time).