The Doctor

The Eleventh Doctor spent the last nine hundred years of his life battling against the universe's deadliest forces to protect the planet of Trenzalore, which caused the Silence to be formed, with the Kovarian Chapter going back in his timeline to prevent him reaching Trenzalore, causing his body to grow old and weak. At the cusp of dying of old age, Clara convinced the Time Lords to grant him a new regeneration cycle, restoring his youth and triggering his eventual regeneration into his next incarnation, greatly changing his future.