Post an anime character that has two personalities .

Lucy/Nyu/Kaede from Elfen Lied

Agito/Akito/Lind from Air Gear. It's possible that he has other personalities.

Juugo from Naruto.

Hatsuharu-kun in Furuba and here's a picture that illustrates that :p

Shiro from Deadman Wonderland. Her alter-ego is Wretched Egg~!

solva from needless

Stein from Soul Eater!!

Ichigo Kurosaki (Bleach)

he have two personalities 1 his own self other .....a hollow's.....he he he he
if his hollow personality is out...........he is deadly...way more deadly.....he he heh eh

Hasta from Nyarko-san: Another Crawling Chaos has two forms. His regular form is a shy girly boy. His true form is a bishounen you probably don't want to mess with.

Kurumi Tokisaki from Date A Live. (If she counts.)

Kurumi doesn't exactly have different personalities, but she can make multiple versions of herself and multiple pasts in different points of time. And there are two specific appearances/personalities that are shown of her.

I would also say Miss Sawako Yamanaka from K-ON has somewhat of a split personality. X3

According to Tyki:

"Having a light side... And a dark side...Is what makes life interesting."

Yogi from Karneval.

Yogi is an eccentric man who always retains a bright disposition. Despite his age, he tends to act like a child; he's child-like, rather than childish. He hates to fight and will do anything to avoid doing so unless his friends are in danger. But when his 'allergy patch' comes off he becomes extraordinarily violent and powerful, changing him into "Silver Yogi", his split personality/alter ego. In this form, he seems to forget who the people he met while he was normal.

Natsuki :D
And his crazier side is Satsuki.

Well I was gonna post Akitoand Agito Wanijima but someone stole my answer, lol.

So, I'll post Kallen Kazuki / Kallen Stadfeld instead

Kawamura Takashi(Taka-san) from Prince of Tennis....he's a soft-spoken person off the court, very gentle and shy, though still outspoken and stubborn when he takes a decision. However, he becomes extremely aggressive and loud once he grabs his racket, or any racket.This change in his personality 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.

Hani Usami! Bunny!
one dislike men & another one dislike women! XD