[Blink]

Name: Shiro
Origin: Deadman Wonderland
Bio:

Spoiler:

Age: 15ish according to Wikia

Backstory and stuff:
Sorae Igarashi didn't want to use her own kid in her and Hagire's experiments, so she adopted Shiro.  Ganta and Shiro became friends when young.  Shiro was extensively 'experimented' on (basically torture), and had to have her limbs literally be sown back on from her body's sudden deterioration that happened at random times.  The experiments caused the creation of a secondary personality known as 'wretched egg'.  Soare eventually killed herself out of guilt.  The 'great tokyo earthquake' was seemingly an attempt at suicide by wretched egg, trying to deteriorate faster than she regenerates.  The mother goose system was created to prevent this from happening again.  Years later, when wretched egg heard of Ganta's location, she escaped, killed all of Ganta's class, and gave Ganta his powers, or 'branch of sin'.  Wretched egg and Shiro interact with each-other separately (and weirdly), with Wretched Egg being somewhat protective of Shiro.  
Ganta goes into the prison, and eventually meets Shiro.  He meets her when he says 'I want to die', and Shiro attempts to kill him thinking that that will please him.  She says that they're friends (knowing his name, which surprised Ganta because he doesn't remember their childhood encounters).
Far later (in the manga), Hagire wants to take over Ganta's body in order to unlock the Mother Goose System, before the attempt of this, Wretched Egg approaches Ganta.  Ganta tries to attack Wreched Egg, but she responds saying that they won't be friends if he kills her, and then she switches to Shiro, and has a mental breakdown and runs.  After Hagire dies, Wretched Egg destroys the Mother Goose system, causing an explosion.

Shiro and Mother Egg eventually fuse.  Shiro runs into Ganta's arms, and the crystals in their chests both explode, leaving Shiro in a coma for a while.

Gifs:

Spoiler: Not actually a spoiler, just pics

Why she should win: She's cute, and she'll kill you and your waifu

Interesting choice! I like the "she's cute and will you and your waifu". #killerwaifus

More specifically, she'll kill you if you happen to encounter her other side or try to kill Ganta

Spoiler:

although her and wretched egg merge at the end so meh

Although considering your wording of 'battling it out', I think Shiro'd win.