Haiyore! Nyaruko-san

Synopsis

Nyaruko believes it's time to invade the headquarters of the culprit located in the city of R'lyeh operated as a theme park. Arriving inside a black castle in the center of an arena they are approached by Nodens, the boss. However, they also confronted by someone from Nyaruko's past, Cthugha, "The Living Flame" and her sworn rival. Fighting Cthugha in another dimension, surprisingly she reveals to have complex feelings for Nyaruko which greatly annoys her. Nyaruko transforms her body in black armor thus returning back through a box she had given to Mahiro. Backing Nodens in a corner, he says that Mahiro was chosen to be part of a TV drama, having Mahiro command Nyaruko to punish him. Nyaruko remains with Mahiro due to her infatuation for him.

(Source: Wikipedia)