Lethe

Lethe (レーテー) is a Greater Devil that appears in Monster Musume: Everyday Life with Monster Girls Online.

Zoological Classification
Greater Devils (グレーターデビル, Gurētā Debiru) are devils that have the bodies of humans in their late teens. Despite the name, Greater Devils are not matured Lesser Devils, but rather their own Devil class. They enjoy dressing in human fashion and venturing into the city at night to play rather malicious pranks.

However, because their actual age is much greater than their apparent age, much like Lesser Devils, they can be out of touch with fashion trends popular among younger people.

Trivia

 * "Lethe", who is the personification of oblivion/forgetfulness/concealment in Greek mythology as well as the name of one of the rivers of Hades.