The government man in the plain suit who kept turning up, until the day the films decided his death was worth an entire team.
Phillip J. Coulson is a high ranking S.H.I.E.L.D. agent and Nick Fury’s longest serving partner, and for the first four years of these films he is the connective tissue: the man who arrives after the thing has happened and asks for a statement. He grew up idolising Captain America and Peggy Carter, and joined the agency Carter founded.
Clark Gregg plays him as unbothered, which is why the audience became attached to him without anyone planning it. He is the first character in this catalogue who was invented for the films rather than adapted from the page, and he has since been written into the comics because of it.
Minor spoiler
Loki stabs him through the heart during the attack on the helicarrier, and Fury uses his death, and a set of bloodied trading cards, to make the Avengers into a team. Fury then has him resurrected with the blood of a dead Kree, which is where Agents of S.H.I.E.L.D. begins: seven seasons of a man working out what was done to him, running the agency, losing a hand, making a deal with a demon to briefly become the Ghost Rider, dying again in Tahiti, and coming back one more time as a Life Model Decoy.
Show detailsHow much of this the films acknowledge
Very little. Agents of S.H.I.E.L.D. is built on the films and the films never refer back to it, which is the central problem of the Marvel Television era and the reason this catalogue marks that tier separately.