This character is mentioned only on an award certificate seen in the "Lockhart's Office" feature on Disc 2 of the Harry Potter and the Chamber of Secrets DVD. The prop award certificate appears to be partially constructed from a membership certificate for a real historical organisation called the Ladies' League for the Defence of the Reformed Faith of the Church of England. This group was founded in 1899 by a Lady Wimborne and advocated against ritualism in the Anglican Church.
↑Presumably Lady Wimborne was of age when she founded the Witches' League in 1899.
↑According to Remus Lupin, nobility and royalty do not exist in the Wizarding World. The fact that Wimborne is "Lady" implies that she comes from Muggle ancestry, making her at minimum a half-blood witch.