Fiel Nilvalen is a character from the Japanese light novel, manga, and anime series No Game No Life. She belongs to the race of Elves, also known as the Elven Guard, and hails from their nation of Elven Gard. Despite her youthful appearance, which suggests a girl around fifteen years of age, her actual age is fifty-two years old. She is characterized by her long, flowing blonde hair, pointed ears, and a tall, curvy figure.

Born into the prestigious Nilvalen family, which holds a hereditary seat in the nation's upper house of parliament, Fiel is the acting head of her house and serves as a proxy senator. Her gentle and soft-spoken demeanor is deceptive, as she possesses a brilliant and highly strategic mind capable of matching the human protagonist Sora in psychological warfare. Her primary and most defining motivation is her deep, protective love for her companion, Chlammy Zell. Fiel considers Chlammy more important than her own nation and has stated she would not care if Elven Gard were destroyed if it meant keeping Chlammy safe.

Her relationship with Chlammy is complex. Due to the elves' practice of slavery, Chlammy is officially Fiel's property. However, the two treat each other as close friends, and their bond is often described by other characters as resembling that of a mother and daughter. Fiel is secretly working to abolish the slave system, a goal that puts her in direct opposition with her own government. To protect Chlammy and advance her plans, Fiel hides her true abilities from her peers. Officially, she is considered a disgrace to the Nilvalen name, having been expelled from the nation's highest magic academy as a "Double Cast" mage who can only use two spells by relying on an amplifier.

In reality, Fiel is an extraordinarily powerful sorceress. She is a Hex Caster, capable of casting and maintaining up to six spells simultaneously, and even has a secret "Seventh Spell" technique as her ultimate trump card. This hidden power, along with her tactical intellect, makes her a formidable player in the games that decide the fate of the world’s races. Early in the story, she uses her magic to help Chlammy cheat in games against the protagonists, Sora and Shiro (known together as Blank). After her deception is uncovered, she and Chlammy form an uneasy alliance with Blank.

In this alliance, Fiel takes on the role of a double agent. She reports a falsified version of events to the Elven Gard senate, using information that Sora has altered with his own psychological manipulations. She also holds a strong personal animosity towards Jibril, a Flugel (an angelic race), because of the historical destruction Jibril caused to the Elven capital in a past great war. Fiel refuses to let anyone except Jibril call her by her nickname, "Fii". As the story progresses, her role evolves from a hidden adversary to a valuable, if reluctant, ally on the path to uniting the world's sixteen races. Her development continues as she becomes more directly involved in major political and military conflicts, using her exceptional magical skill, deep-seated cunning, and fierce loyalty to Chlammy to navigate the struggle for control of Elven Gard and its place in the new world order.