This unidentified witch was the witch who gave Fiona her curse. She has only been referenced and never physically seen, but she is known to have recited a poetic spell when turning Fiona into an ogre at night.
The poem goes as follows:
By night one way
By day another
This shall be the norm
Until you find true love's first kiss
And take love's true form
- Even though she is unidentified, she has the most influence in all 4 Shrek films.
- There have been multiple theories that the witch might have been the Fairy Godmother, but it's all up for speculation.