Yūgao Uzuki

is a member of the Konoha's ANBU, and was Hayate Gekkō's lover.

Chūnin Exam arc
After Hayate was killed by Baki, Yūgao swore to avenge him.

Invasion of Konoha arc
As it is unlikely she knows who killed Hayate, it is possible she meant she would get her revenge through fighting against the invasion.

Fire Temple arc
Yūgao briefly appeared during the Naruto: Shippūden anime, catching Sora spying on Danzō's interrogation of Tatsuji. They apprehended him and turned him over to Tsunade.

Other media
Yūgao is a playable character in Naruto: Clash of Ninja Revolution 2. In the game, she possesses techniques similar to Hayate's. While it is unclear if she uncovered the identity of Hayate's killer, her battle cry when using Dance of the Crescent Moon when fighting Baki in the game changes to, "Revenge ... for Hayate!" She has a small role in the game's storyline, appearing in the antagonists' hideout to save Naruto, Kakashi, Gaara and Sakura from a group of missing-nin, stating that the ANBU would take care of the situation. She also has a team special with Kakashi, most likely due to the fact that she refers to him as her superior.

Yūgao also appears as a playable character in Naruto Shippūden: Gekitō Ninja Taisen! EX 3 and Naruto Shippūden: Clash of Ninja Revolution 3. She is also the model of the female ANBU party member in Naruto: Path of the Ninja 2, but the player can bequeath her with any name they please.

Trivia

 * "Uzuki" is the old name for the fourth month of the Japanese lunar calendar, while "Yūgao" means "moonflower".