Dragon Volume 6

Being born from nothingness, Ophis rarely shows any sort of emotion, only wanting to get rid of the Great Red and return to the Dimensional Gap to obtain "silence". Issei describes Ophis as an honest and pure Dragon who is naive. Azazel indicates Ophis has changed because of her interaction with humanity, but how is never described.

Some time in the distant past, Ophis was born in the Dimensional Gap, and lived there as its home. Eventually, long ago, Ophis left the Gap, but when it attempted to return, found that the Great Red had taken up residence there. Before assuming her current form, she used a different guise in the form of an elderly man. At some point before the start of the series, Ophis became the leader of the Khaos Brigade.

Ophis' name was mentioned in Volume 4, in which Ddraig calls her "the strongest existence".

She made her official appearance in Volume 6 in the Underworld to look at the Great Red before retreating.

In Volume 13 Extra Life, Ophis is seen playing with Rassei whom she has developed a close relationship with, even going as far as stating that she will train the young Dragon.

As her name suggests, Ophis has the ability to manifest her powers in the form of snakes. These snakes have the ability to increase a person's power tremendously by consuming it. Another type of snake she creates can be used to bring out a Sacred Gear's full potential. While this can potentially lead them to achieving Balance Breaker more quickly, it can also break the Sacred Gear, killing the host.

According to Ddraig, her true abilities are stronger than the two Heavenly Dragons combined. So much so that when her powers are reduced to level twice as powerful as the Heavenly Dragons, she feels "weak."

Ophis is a true shapeshifter, able to alter her size, apparent age, physical attributes/characteristics, and even her apparent gender.

Ophis can 'bless' people, granting them certain advantages. These blessings don't seem to be of a type to negatively affect Devils. With Asia, the blessing has increased her compatibility with Dragons, while with Irina, it has increased her luck.

(Source: Highschool DxD Wiki)