Yuri Nikaido

Yuri Nikaido  (二階堂 由梨)

A born sadist, Nikaido is Keima's homeroom and Japanese teacher. She has been ignoring Keima ever since he told her that there are 5012 games that are more fun than her lessons.

Nikaido is also a school senior of Jun Nagase.

Age: 22+ Height: 170 cm Class: 2-B

(Source: Wikipedia)

Major spoiler (Don't open if you didn't read manga):

In the later chapter, she was revealed to be one of the partner of demon called 'Akari'. This might have been foreshadowed from the start of the series, as she always had collar around her neck like Keima.

Although she is a human and apparently borrowing all her abilities of capturing from Akari, she is shown to be extremely powerful. She was able to capture level 4 spirit without a single difficulty even though it was not able to be handled by entire run-away spirit squad from the district.

In chapter 250 it revealed that Dokurou "Human" has a resemblance of Yuri Nikaido and that is why she has the Tape collar to keep vintage from finding the girl with the goddess, so she became the decoy for a while until Keima meet Elsie. Also she said she will always be with Keima protecting him in a distance...like Yuri did in some of the chapter helping him in secret. So for those who couldn't figure it out they are the same person. (Source: AkaksukiUlquiora)