I basically just trashed my other answer, but you can always see it again in the edit history: as of now she is still an unknown assailant.
Background- Still unknown (partially):
Evermillion is a huge mystery. Her origins, how she came to be sealed in Honoka, why she was sealed, and even what she actually is; are completely unknown (she referred to herself as a demon with Honoka, though it's possible she was being metaphorical). Sometime in the past she was sealed inside of Honoka Takamiya with a five layer barrier preventing her from escaping. She seems to have some history with Medusa.
Who is she- Still unknown (partially):
Evermillion (エヴァーミリオン?), fully named White Princess Evermillion
(白姫エヴァーミリオン Shirohime Evamirion?), is an entity of unknown origin
sealed inside Honoka Takamiya. She is addressed as the White Princess
by various characters. A total of five seals were placed on her.
Oh yeah, and theres scheduled to be an OVA as of 2015 per Wikipedia too.