Natasha Romanoff/Black Widow
Nat's journey in Endgame concludes a bit earlier than everyone else's. After the Avengers figure out how to use Pym Particles to time travel, they separate into groups to go back in time and collect the Infinity Stones. Hawkeye and Black Widow are assigned to grab the Soul Stone, which can only be retrieved by exchanging a life for a life. The two struggle with one another as both try to save the other from jumping. In the end, Nat succeeds in getting Clint to let her go and plummets down the cliff to her death.
Marvel is planning a standalone Black Widow movie, so this isn't the final time we'll see Nat, but it is the end of her journey with the Avengers.