This is Kurai. For a few hundred years, she was Giratinas trusted companion. She loved the Reverse World when she was young and often followed Giratina inside whenever she could. The Renegade Pokémon took a liking to her and decided to raise the young Pokémon.

One day, however, Kurai realized that Giratina was not the hero she had thought him to be. He cursed her to be stuck in the Reverse World until he allowed her to leave, but this gave her the ability to survive there indefinitely. Normally, she would have died, however the curse keeps her alive.