Kiseki Kusanagi

Kiseki is the younger twin sister of Takeru Kusanagi and is one of the last surviving members of the Kusanagi Clan. She is the current host of the Hyakki Yakou, the manifested form of countless demons that is the product of the Kusanagi Clan's inherited curse. For the longest time, she was imprisoned deep inside an Inquisition holding facility where she was both experimented on for her immortality and restrained to keep the world at large safe.

Kiseki is extremely strong willed as she continued to remain sane after being killed repeatedly throughout her years in captivity. With Takeru being the only person she's ever interacted with, she is understandably shy and fearful of others. However, she still exhibits inherent kindness to those who treat her like a human being. As she grew older, Kiseki began to feel the immeasurable guilt in how her very existence was responsible for so much death and coupled with her constant, physical torment, her only wish was to be killed.

After Takeru goes back against his word to end her suffering, Kiseki becomes psychotically obsessed with destroying the world in the hopes that Takeru would kill her.

(Source: Taimadou Gakuen 35 Wiki)