With Avengers: Endgame on the rise, fans are speculating the safety of their favorite characters. Creators and trailers have hinted at the serious sacrifice the surviving Avengers will have to make to save everyone. One of the characters most in danger, according to the internet, is Captain America. Not only has the character lost his closest friend and is more motivated than ever, but in the real world Chris Evans' contract is up after the film. This might be the last film fans will ever see Steve Rogers in.

While heroism helps motivate Cap to commit to fatal sacrifices, there's another piece in play: Bucky Barnes. Right after Thanos' fateful snap, The Winter Soldier disintegrated before his best friend's eyes. Throughout the Captain America films, Steve has shown the lengths he's willing to go to in order to protect Bucky. They're friends, family, and the most important person in each others' lives. Losing Bucky (again) is something Steve doesn't want to do.

It's not every day people get to have a best friend for over a hundred years, after all.

This bond has spanned decades of comics, several movies, and animated stories, so it's a pretty deep-seated part of the Marvel universe. But unless a fan's looked at it all, there are important parts of their relationship that could pass right on by.

Here are 25 Things You Don't Know About Captain America And The Winter Soldier's Relationship.

Bucky Started Out As A Kid Sidekick

Most people know Bucky Barnes as The Winter Soldier, the hunky childhood friend of Captain America. While both men were lost to time, Barnes was the tortured soul used by Hydra to further their evil machinations.

However, in the comics, Bucky started out a young ward to Steve Rogers, a kid who helped him in his superhero adventures. The teen idolized the man with a shield and they became a team. Unfortunately for him, though, the partnership ended with Bucky's abrupt passing during Avengers #56, in 1968.

It wasn't until 2005 that Hydra warped the young Barnes and turned him into The Winter Soldier fans know and love today.

Both Men Flirt With The Black Widow

Captain America comfronts Black Widow in Captain America: The Winter Soldier

Whether it's the movies or comics, Black Widow has had a large variety of relationships. Two men that seem to pop up in her life over and over again, though, are Steve Rogers and Bucky Barnes. Early on in the MCU, it seemed like they were setting up a Natasha/Steve pairing but never followed through. And elsewhere? In Next Avengers: Heroes Of Tomorrow, Cap and Widow even have a son named James Rogers.

Bucky and Natasha have quite the history, though, too. While under Hydra control, Black Widow was not only a fellow agent, but his trainer at times. Canonically, during their brainwashed time together, they fell in love. It's unknown if the MCU is keeping that part of its canon, though.

Bucky Ended The Lives Of  Avengers' Family Members

Howard Stark wears sunglasses in Agent Carter TV series.

During his time as Hydra's Winter Soldier, Hydra coerced him into doing a lot of things his former self would never agree to. Once a war hero, they changed him into the perfect assassin, the kind that could expertly pull off a hit and disappear at a moment's notice. His assassin career included taking out high-profile targets like dignitaries, scientists, and even JFK himself.

Captain America: Civil War started because of Bucky's nefarious past, when he was blamed for the bombing of Wakandan King T'Chaka. While that was discovered to be a lie, another horrible target of The Winter Soldier was revealed: Tony Stark's parents. On Hydra's orders he eliminated them, leaving their son alone in the world at only 21.

Without Steve, The Winter Soldier Wouldn't Be Alive

Regardless of who the real Bucky is, as The Winter Soldier, he committed a lot of terrible crimes for Hydra. When he regained control of his mind, of course James Buchanen Barnes despised what he'd done.

However, that didn't stop people from wanting to bring The Winter Soldier to justice. If it wasn't for Captain America's fierce loyalty, Bucky likely would have been destroyed by Natasha in her initial investigation with Nick Fury. Or if Barnes in any way came to his senses, the Sakovia Accords would have outed him and put him in danger. Steve's protection the only reason he ever made it to Avengers: Infinity War.

Young Bucky Stays Gone

In the current comic climate, it's rare for lost characters to stay in their graves. In the '60s, though, Marvel comics did something wild and revolutionary by ending young Bucky Barnes' life and keeping it that way. Over the decades, losing him has been a constant source of anguish for Captain America. Furthermore, some creators thought it was important to show the danger of including children in heroics.

Though Barnes has returned as the Winter Soldier, the younger version of him has stayed gone. Marvel comics have only included him as an adult since his resurrection in 2005. Out with eager child, in with brooding, troubled ex-Hydra agent.

In Some Comic Runs, Bucky Is Captain America

Throughout comics' run, artists have always played with passing down the mantle of certain titles to different characters. Bruce Wayne isn't the only Batman. "Thor" has now passed onto several new Gods. Even the Avengers have evolved into the West Coast Avengers or the Young Avengers. Captain America is no different. While Falcon (AKA Sam Wilson) is a common candidate, he isn't the only friend of Steve's to take over the mantle. Bucky Barnes himself has become Captain America when Steve has stepped down. Steve even recommended him for the job.

Barnes' version of Captain America has a bulletproof suit and pistols. While effective and dedicated to heroism, his past still leaves a dark stain on his reputation and his mind.

The Captain America Creator Named Bucky After His Childhood Friend

Captain America and Bucky Barnes comic

When it comes to character creation, it's not uncommon to name characters after people. Captain America creator Joe Simon gave his good childhood friend, Bucky Pierson, that honor when concocting Steve Rogers' sidekick. Though he was Cap's young protege, his bond with Bucky meant a lot to him. Losing the young boy haunted Steve for decades.

In the MCU, Bucky's identity is a bit more meta to its origin. This Barnes is a golden boy athlete and best friend of Steve, instead of a younger kid. That fits Simon's old friend's description. After all, the real Bucky was captain of the high school basketball team.

Both Men Are Over 100 Years Old, Chronologically

Winter Soldier File

Of all the Avengers (excluding Thor,) Steve and Bucky are by far the oldest members. Hailing from the 1940's, these WWII soldiers are brave, heroic men. With time, though, they've gained a lot of scars and pain. Steve has lost too many people to time. Bucky lost himself to Hydra. However, both men can bond over the people they used to be  and the past they once had.

No matter the hard times between them and the enemies they've faced, they can get through all of it together. After all, Steve remembers when Bucky was a hero and Bucky remembers when Steve was just a man.

Bucky Started Out As The Perfect Golden Boy

Whether fans are talking sidekick boy Bucky Barnes or the man who became The Winter Soldier, either one of them started out a golden boy. Strong, athletic, and good-hearted, Bucky has always been a pinnacle of pride and heroics. His best friend Steve admired that in him, and so did the original Captain America that took young Barnes in as his sidekick.

Hydra may have done a lot of things to the war hero, but that can't take away who he was. There is a part of The Winter Soldier that will always want to be a good hero. It's why Steve and him became so close.

They're Both Fugitives

After the Sakovia Accords, life changed irreparably for both WWII soldiers frozen in time. Wakanda had to re-freeze and heal The Winter Soldier. Captain America had to go on the run with Black Widow and others because of his Accords opposition. Under the eyes of the American government, both men are fugitives and criminals. If they were caught (pre-Infinity War,) the U.S. government would have sent both of them  to the terrible meta-human prison, The Raft.

The past few years have been spent in hiding from the very people they spent decades wanting to protect.

With the world in chaos after Thanos, though, there likely will be exceptions made for Cap and others.

In An Alternate Universe, Bucky Commands Steve's Unit And Becomes White Skull

Most Marvel storylines follow similar origins, but sometimes creators get, well, creative. In an alternate timeline story, Bucky and Steve's relationship is completely altered. Instead of mentor/mentee or childhood friends, they meet as grown men in the army. Barnes in Steve's commanding officer in the American Civil War and leads him through battle. Another big change is that "Buck" doesn't become The Winter Soldier or simply lose his life. Instead, he becomes The White Skull, fighting against Captain America.

Also, in this timeline, Lincoln never was assassinated and Native American culture is revered by the whole country, wildly different from the real events of the time period.

Marvel's Changing Timeline Protects Their Origin Story

Chris Evans as Captain America Steve Rogers and Sebastian Stan as Bucky Barnes Winter Soldier in Civil War

Other than a few, off-hand stories, Captain America and The Winter Soldier are always tied to WWII. Their origins are rooted in the world-wide conflict, building them into the men they are today. Because of this, Marvel's sliding timeline rarely affects their origins.

This means that, while Peter Parker's teen style may evolve with the changing decades, the core of Steve and Bucky's past remains unmoved. After all, their origins aren't just the 40's because that was an arbitrary choice. That time was when these good, young men became the heroes they are today. It's very similar to the Wonder Woman's involvement with WWII and Steve Trevor in the DC comics.

The U.S. Government Honors Their WWII Service

Bucky Barnes Smithsonian Memorial Exhibit The Winter Soldier

Both Steve Rogers and Bucky Barnes, officially, lost their lives in the line of duty. They helped protect the Allied forces against Hydra during WWII and were heroes for it.

Between WWII and current day, the U.S. government found ways to honor the lost heroes that day. Captain America was commemorated and Bucky Barnes had his own memorial. Though it was a morbid sight, seeing America honorably display his history helped remind The Winter Soldier of who he once was.

While they both ultimately didn't perish in WWII, it was still impressive to both receive such high honors for their service. They even got to see the results of those actions in the future, for better or worse.

Captain America Replaced Bucky With A Doppleganger

Rick Jones Marvel

While Bucky Barnes was dear to Cap's heart, he wasn't the guy's only sidekick. Most notably he also teamed up with Rick Jones, the spitting image of his old companion. Though his hair was a little longer and he was a lot more annoying, the strong face, brown hair, and intense eyes were all there.

However, looking like Bucky wasn't what made Cap team up with Rick. Rick inserted himself by Steve's side and the hero agreed because of his soft spot for Bucky. No matter how similar they looked, Rick simply just wasn't the same. He took Bucky's costume and name without Cap's permission and was a jerk about the whole sensitive situation.

They Have A Complicated Relationship With Wakanda

Bucky Barnes aka the Winter Soldier in Wakanda at the end of Captain America Civil War

When The Winter Soldier and Captain America first encountered Wakanda, the Black Panther was hot on their heels, wanting to make Bucky suffer for his supposed crimes. After all, he was wanted for the loss of The Black Panther's father, King T'Chaka. The pair had to evade Wakanda at every turn.

However, when Investigations revealed Bucky didn't cause T'Chaka's passing, Wakanda grew softer to the duo. They understood Steve's desire to protect the tortured Winter Soldier. They even helped him keep Barnes away from American forces and heal him. Wakanda is the only reason he was able to fight by Steve's side ever again.

Pre-Captain America Transformation, Steve Only Trusted Bucky

Civil War Steve Rogers by Christian Cordella

When MCU Steve Rogers was a kid, he was treated poorly. He wasn't always the big, powerful, beefy man people know Cap for. The hero used to be skinny, weak, and heroic, despite his physical deficiencies.

Between losing his parents and the copious bullies he fought off, the only person he had anymore was his best friend, Bucky Barnes. Barnes not only supported Steve, but he protected him against the darker parts of the world. Despite his friend's good heart, Bucky was vehemently against Rogers joining the army. After all, he didn't want to lose Steve just because he wanted to be a hero. Even though he trusted Barnes, he defied his friend.

Lucky for Bucky, Steve became the hero he always dreamed of being.

They Both Liked Peggy Carter

Captain America Steve Rogers Peggy Carter

From the second Steve met Peggy Carter, he was enamored with her brilliance and indomitable spirit. Over their time working together, the two began to foster feelings for one another. Steve became Captain America, this shining and true hero that Peggy admired. And Peggy? She encouraged Steve to be the man he was always meant to be.

When Bucky finally met Steve again, the scrawny kid he once knew was very different. The second Peggy joined them, he was also charmed by her good looks and strength. However, he was startled to realize he wasn't the one catching all the girls' attention anymore. She only had eyes for Steve.

Both Men Rely On Metal, Advanced Tech Attachments

At their core, both Captain America and The Winter Soldier are super-serumed warriors with advanced tech. While Bucky relies on his robotic arm, Steve relies on his powerful, bulletproof shield. Not only are they staples of their fighting styles, but also signature tools that define their hero personas.

When either men got their metal attachments, it changed them as people. With Hydra, Bucky's robotic arm marked his assimilation into their ranks. Meanwhile, Cap's shield became his symbol of protection and strength.

Though these items don't define them, they do represent their past, who they are as heroes, and who they are as people.

They Will Sacrifice Their Lives For One Another

Bucky Barnes and Steve Rogers in Captain America The First Avenger

Despite their initial physical differences, Bucky and Steve always remained fiercely dedicated to one another. They would do anything to keep each other safe. With this in mind, it's why Bucky was cruel about the army to try to protect Steve and sacrificed himself during WWII. Furthermore, it's the reason Cap risked life and limb to save Barnes from Hydra facilities, the Red Skull, and eventually the U.S. government itself.

Time and time again, they chose each other over anyone else. After all, they were best friends in the 40's, and now the only family they have in modern day. Without each other, they'd lose a part of themselves that both men cherish: the friendship that encouraged them both to become heroes.

Steve And Bucky Met As Children

As a little kid, other kids his age bullied and attacked Steve. Despite being popular and unaffected, Bucky stepped in to protect the young Rogers. Just because he had it all didn't stop him from believing in protecting others. From this moment on, the pair became fast friends and stuck by each other's side all the time.

When Steve's mom passed away, Bucky was there to support, comfort, and protect the young Rogers. Though they disagreed on Steve's abilities, they became family. Between hard times and good ones (like sneaking to Coney Island to buy hot dogs or gifts for Dolores,) their friendship became the most important thing in both men's lives.