Sister Nivedita, original name Margaret Elizabeth Noble (1867—1911) was an Irish-born schoolteacher who was a follower of Swami Vivekananda and became an influential spokesperson promoting Indian national consciousness, unity, and freedom.
Nivedita met Vivekananda when he visited England in 1895, and she was attracted to the universal principles of Vedanta and to Vivekananda’s humanistic teachings. Nivedita also made notable efforts to serve the poor of Calcutta and Bengal during times of plague, famine, and floods there. Nivedita’s tireless activity, austere lifestyle, and disregard for her own welfare eventually caused her health to fail, and she died at the age of 44.
Here is a book on Sister Nivedita.