Elaine, mostly known as the Lady in the Lake, is a powerful water-spirit of a young girl who drowned in a lake and, who's immense amount of power, was able to manifest herself as another entity that bonded itself with the water.
When manifesting herself in her human form, the Lady in the Lake is shown to be a young girl, around the age of 9-12, with short white hair and dark eyes. However, when she meets with Sadow, she is fairly older, with longer white hair and blue eyes.
She is quite caring and compassionate, and shuns things like fire and malice. She is the one who gave Arthur his sword, Excalibur, in a reforged form after he had lost it in his fight with Mordred. While she does side with Merlin in his quest to protect Brittannia, she disapproves of his manipulative nature.