Tony Stark's evolution is the defining arc of the Marvel Cinematic Universe

Tony Stark/Iron Man, played by Robert Downey, Jr., is not only the most important character in the Marvel Cinematic Universe, with an unmatched five leading parts and pivotal roles throughout.

He's also the most interesting character, due to inherent aspects of his story, choices by the filmmakers, and Downey's complex portrayal.

While other characters appear static - Steve Rogers is heroic before and after becoming Captain America; Bruce Banner is a loner before and after becoming the Hulk; Thor is a bro god from beginning to end; Natasha Romanoff's dark past is only alluded to; Clint Barton is grounded whenever he's not being mind-controlled - Stark is ever dynamic.

Stark's centrality has been helpful, offering a popular human element to tie everything together. At the same time, it poses a risk: Downey is under contract for only two more movies, and he reportedly demanded a fortune for his last extension.

We've highlighted his character evolution below, with some spoilers.

At the start of "Iron Man," Tony Stark is a selfish genius weapon maker.

In a time of crisis, he makes something that can truly help the world.

Stark shows valor as he keeps his creation away from dark forces. He signifies his transformation when he announces, "I am Iron Man."

But this heroic transformation leaves him cockier than ever, as seen in "Iron Man 2."

He gets humbled: first, by a rival engineer; second, the possibility of death; third, his own embarrassing behavior; fourth, the revelation of his father's secret accomplishments.

Stark completes his transformation into a selfless hero when he sacrifices himself to save the world in "Avengers."

When he survives the sacrifice, Stark's selflessness becomes an obsession. He can't stop working, building iteration upon iteration of Iron Man, while his personal relationships suffer in "Iron Man 3."

When forced to save the day without his suits, however, he finally realizes that he, not the suit, is the hero. It now has new meaning when he says, "I am Iron Man."

Stark, an engineer unleashed, focuses on his biggest project, an artificial intelligence capable of saving earth from a threat only he understands in "Avengers: Age of Ultron."

Has Stark caused more harm then good? An intelligent robot he unleashed, Ultron, tries to destroy the Avengers with the help of two super-powered siblings whose village was destroyed by Stark weapons.

Stark sticks to his guts and helps create an android, Vision, who saves the world and becomes mankind's best hope for the future. Stark is not "worthy" to lift Thor's mystical hammer, but he has created someone who is.

Downey may be under contract for only two more movies, but Marvel will certainly get a lot out of him. Next year's "Captain America: Civil War" will feature Stark as an anti-hero in another promising transformation.

"The clues are in Ultron about where we might find him next," Downey told Empire. "But what would it take for Tony to completely turn around everything he's stood for?"

Then we'll have the two-part climax of the Avengers, and you can bet Stark will play a central role.