The Doctor's saviour

The Doctor's saviour gave her life for the Eighth Doctor at the very end of the Last Great Time War: he saw her skeleton crumbling to dust right before he destroyed Gallifrey using the Moment. After doing so, he initially believed that using the Moment had made his existence fixed, preventing him from ever regenerating again; however, as he began to regenerate, he realised that in her last kiss, she had passed on the Restoration to him, thereby "resetting his life cycle" and saving him one last time. (PROSE: Doctor Who and the Time War)

In another account, however, these events did not take place before the Doctor's theft and use of the Moment, as it was instead the War Doctor, alone, who stole the Moment from the Omega Arsenal; upon becoming the Ninth Doctor, the War Doctor then muttered "Of course" to himself, not appearing surprised that he was regenerating once more. (TV: The Day of the Doctor)