10 Small Details You Missed In Tenet
2. Trusting Neil
Wonderfully played by Robert Pattinson, Neil is one of the movie's most interesting characters. He feels like a callback to pulpier spy fare and acts as a solid foil to the Protagonist's more straight-laced approach.
It has been frequently posited that Neil is a future version of Kat's son, due to their similar haircuts and their connections to the Protagonist. This theory may not hold water due to causality having wildly different effects on both characters by the end of the movie, but that does not mean that Neil does not have any more layers to him.
Towards the end of the movie, the Protagonist sees a red strap on his comrade's bag and realizes that he was the individual that saved him in the opera house. The strap may be a reference to the myth of Theseus and the Minotaur.
To face the beast, a woman named Ariadne gives him a sword and red string. However, she cannot flee the maze that she was trapped in with said beast, mirroring Neil's ultimate fate to be trapped in the time loop.