saviour

noun
  1. 1

    A person who saves someone, rescues another from harm.

  2. 2

    A child who is born to provide an organ or cell transplant to a sibling who has an otherwise fatal disease (used in combination, with "sibling", "baby", "child", "brother", "sister", etc.)

Source: https://en.wiktionary.org/wiki/savior, https://en.wiktionary.org/wiki/saviour