Najenda is the head of the assassin group Night Raid. Her background story, regarding the events which leads to the amputation of her right arm and the lost of her right eye, is shown in chapter 39 of the manga, right after the events in Kyoroch in the previous chapter (which corresponds to episode 19 of the anime). However, her story is not shown in the anime.
Why didn't the anime show her background story? I think her background story complements the rest of the series very well and make it easier understand.