Shiro

Despite her seemingly harmless appearance, Shiro harbors a dark secret; she possesses two personalities and her second one is the 'Red Man' Ganta is searching for, who is also the 'Wretched Egg', the source of the Branch of Sin infection. Thanks to the Mother Goose mainframe, Deadman Wonderland's supercomputer, which constantly emits a lullaby audible only to Shiro and Tamaki's father, Shiro's true personality is repressed. When the lullaby is turned off, Shiro becomes a murderous, smirking maniac capable of massacring whole squads of armed men with little effort. It's at these times that she dons a crimson colored full body restraint, which gives her a taller, masculine appearance, hence Ganta's inability to recognize her (later it's revealed that this costume was made after the 'Aceman', a superhero character in a cartoon Shiro and Ganta used to watch together as kids). Shiro cares greatly for Ganta and constantly seeks his approval. She is willing to do anything that Ganta asks her as long as it will make him happy. Even when in her 'Red Man' state, she still seems to care for him, as it was implied that she butchered Ganta's classmates just so that he would be sent to Deadman Wonderland, where she was also living.

(Source: Wikipedia)