Clint Barton spends most of the film under Loki's mind control until Natasha fights him and frees him.
To get the Soul Stone, one of them has to give up their life. Each tries to be the one, and Natasha dies so Clint can bring the Stone back.
In 2012 Natasha saves Clint's mind. In Endgame she saves his life, at the cost of her own.