Who is the robe lady in Harry Potter?

You're likely referring to Madam Malkin, the owner of Madam Malkin's Robes for All Occasions. She is a witch who makes and sells robes for students at Hogwarts School of Witchcraft and Wizardry.

She is most notably remembered for helping Harry Potter get fitted for his Hogwarts robes at the beginning of *Harry Potter and the Sorcerer's Stone*.