Jibril

Jibril is the youngest and strongest of the Flügel race. Her love toward knowledge started even before the declaration of the Ten Oaths. She is against the book sharing law created by the Council of Eighteen Wings and left Avant Heim, her homeland, to win the National Library of Elchea from the previous king in a game. When Sora heard what happened to the library, he challenged Jibril in order to gain knowledge of the other kingdoms. When she lost the game, she became Sora and Shiro's servant since she had bet everything she owned, including herself, claiming that knowledge was worth her own life. However, she was allowed to keep the library and maintain it although she now must share it with the rest of Imanity. Jibril thus took the siblings as her masters and over time she began to understand and accept Sora's belief in the potential of humans.

According to Azriel, Jibril was given the gift of imperfection by Artosh. This gift is what Artosh believed would allow Jibril to find the answer to the Flügels' continued existence should he disappear (which was, in fact, the case).

Unlike Steph, Jibril is happy to become the siblings' underling, and is shown to have growing feelings toward Sora.

(Source: No Game No Life Wikia)