12. Phil Coulson
As Nick Fury worked to put his Avengers Initiative together, Agent Phil Coulson was with him every step of the way. He witnessed the arrival of Carol Danvers that inspired the programme, and he interacted with almost every member of the original team before they were ever a team.
Nat and Clint were his colleagues at SHIELD, he dealt with Tony as he was battling both Obadiah Stane and Ivan Vanko, and he was almost killed by the Destroyer when Loki sent it to Earth after Thor. Not to mention his collection of Captain America trading cards.
There is no doubt that Phil Coulson was an absolute badass, and had no hesitation in standing up to a demigod with a weapon he didn’t even know the function of. This was ultimately his undoing, but his death was able to have more effect on the Avengers than he potentially ever could have in life.
The six heroes were far from a team, until they had something real to unite them and force them to put egos aside. That wasn't the fact that Earth had been invaded, it was the death of Phil Coulson. They put their petty squabbles aside to come together and avenge him, and avenge him they did.