The Great Fairy of Wisdom resides within Death Mountain Crater. She grants Link an increased Magic Meter if he visits her. Whenever Link visits her fountain, she will fully replenish his hearts and Magic Meter.
The Great Fairy of Wisdom lives in Snowhead. After Link collects all of the 15 green Stray Fairies from Snowhead Temple, he can bring them back to her to restore her original form, and will be rewarded with an increased Magic Meter. She will fully restore Link's magic meter and hearts when he visits her fountain, regardless of whether she is whole or shattered.
In the 3DS remake, she lives in Woodfall.