The Avengers are Earth's Mightiest Heroes. Who are the mightiest heroes on Earth? Well, based on the team's roster, just about everybody. These days, it seems like almost every Marvel hero has been issued an Avengers ID card. When they first formed in "Avengers" #1 (1963) by Stan Lee and Jack Kirby, the Avengers was a small operation. There was one team, with five members.

RELATED: 15 Heroes Who Never Should Have Been Avengers

Since then, however, there have been West Coast and Great Lakes Avengers, along with New, Mighty, Secret and Uncanny Avengers. That's a lot of teams, and that's a lot of spots to fill. That being said, there are still a few Marvel characters that have never officially joined an official Avengers team. Here are 15 characters we could think of who never officially joined up for some avenging.

15 JEAN GREY

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As an omega-level mutant, Jean Grey is one of the most powerful heroes in the Marvel Universe. It also helps that she is often bonded to the Phoenix Force, a cosmic entity of immense power. She first appeared in "X-Men" #1 (1963) by Stan Lee and Jack Kirby, and she is one of the most recognizable members of the team. As one of the original five X-Men, she's remained with the team throughout her active career.

The original Jean died (for the final, and seemingly permanent, time) in "X-Men: Phoenix - Endsong" (2005) by Gregs Pak and Land. Currently, the teenage version of Jean Grey has been brought to her future (the modern Marvel Universe), along with the other original X-Men in "All New X-Men" by Brian Bendis. Seeing her future in particular, and all of the death it included, caused Jean to doubt her standing with the X-Men, although she's ultimately stayed with her fellow mutants. She's on friendly terms with the Avengers, but she's never joined their ranks.

14 ADAM WARLOCK

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Adam Warlock has a complicated history. He first appeared in "Fantastic Four" #66 (1967) by Stan Lee and Jack Kirby as a character simply called Him, a perfect human being artificially created by a group called the Enclave. He rebelled against his creators and after a conflict with Thor, retreated back to his cocoon. In "Marvel Premiere" #1 (1972) by Roy Thomas and Gil Kane, he was reborn as Warlock, and would eventually come into possession of the Soul Gem, one of the stones that would later make up the Infinity Gauntlet.

Warlock would go on to play vital roles in some of Marvel's biggest events. He saved the universe from Thanos and Nebula in "The Infinity Gauntlet," and when the evil version of his personality, Magus, later threatened the cosmos in "Infinity War," Warlock once again saved the day. Warlock returned years later to defeat Ultron in "Annihilation: Conquest." With all of his power, however, Warlock seems too busy protecting the entire galaxy to stay on Earth with the Avengers.

13 ELEKTRA

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As one of Marvel's deadliest assassins, Elektra has been both a hero and a villain. First appearing in "Daredevil" #168 (1981) by Frank Miller, Elektra has a dark and mysterious past. She was trained in the martial arts by Stick, who hoped that she would become a member of the Chaste, but she instead was driven to that group's rival, the Hand, by the darkness in her soul. Since then, she's sided with Daredevil and Wolverine, and has even been hired by S.H.I.E.L.D., although she's never joined the Avengers.

She did technically play an important part in Avengers history, however. In "New Avengers" #31 by Brian Michael Bendis and Leinil Yu, the Avengers travel to Japan to save Echo, who has been targeted by the Hand (led by a seemingly evil Elektra). During the fight, however, this Elektra was killed and revealed to be a undercover alien Skrull. This kicked off the "Secret Invasion" storyline, although Elektra didn't reappear until after the invasion was thwarted. Since then, she's mostly avoided the Avengers, although she did briefly join the Thunderbolts.

12 GUARDIANS OF THE GALAXY

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While other members have come and gone, the Guardians of the Galaxy, in their current incarnation, consist of Star-Lord, Gamora, Drax, Rocket Raccoon and Groot; at least for the last several years. This particular team first started coming together during the events of "Annihilation" and "Annihilation: Conquest," first officially appearing in "Guardians of the Galaxy" #1 (2008) by Dan Abnett, Andy Lanning and Paul Pelletier. They spent the majority of their initial run dealing with evil outer space cults, invading alternate universes and a resurrected Thanos. Standard stuff, really.

While many members had individually been to Earth, this version of the Guardians first teamed up with the Avengers in "Avengers Assemble" #4 (2012) by Brian Michael Bendis and Mark Bagley to defeat Thanos. Since then, several Avengers have joined the Guardians (Iron Man, Captain Marvel and Agent Venom), but none of the Guardians have decided to make it official with the Avengers. They are usually willing to swing through the solar system and help out when it's needed, however.

11 CYCLOPS

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As the first official X-Man, Scott Summers has always held an elevated status in the Marvel Universe. He first appeared in "X-Men" #1 (1963) by Stan Lee and Jack Kirby as a shy teenager, although he eventually grew into a capable and authoritative leader. With only a few brief lapses, he's led the X-Men against their fiercest foes and saved the world countless times over.

As the leader of the X-Men, this didn't give Cyclops a lot of time to go off and join other teams. Also, while the Avengers and the X-Men were allies and were mostly on good terms, there wasn't too much crossover between the teams until the formation of the Avengers Unity Division in "Uncanny Avengers" #1 (2012) by Rick Remender and John Cassaday. The team, combining Avengers and X-Men members, was formed as a response to Cyclops, who had become more militant in his views against human and mutant coexistence. Considering that the team was formed to combat his specific agenda, Cyclops was never offered membership.

10 MISTY KNIGHT

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While working as a lieutenant for the NYPD, Misty Knight lost her arm, attempting to defuse a bomb. Luckily, Tony Stark heard her story and decided to have her outfitted with a bionic replacement. Leaving the police force, she would go on to set up a private detective agency with her friend Colleen Wing, which was named Nightwing Restorations Ltd. She ran across Iron Fist in "Marvel Premiere" #21 (1975) by Tony Isabella and Arvell Jones, and has been an ally of the Heroes for Hire ever since.

The closest Misty has ever come to being an Avenger was in the alternate reality of the "House of M" storyline. In that world, Luke Cage formed the Avengers to protect the humans in a world ruled by mutants. Misty originally joined as an undercover operative for the NYPD, but eventually defected from the mutants and became a genuine member of Luke's team. Unfortunately, being an alternate reality, this doesn't officially count towards Avengers membership.

9 SILVER SURFER

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When Galactus first came to Earth, his arrival was forewarned by the arrival of the Silver Surfer in "Fantastic Four" #48 (1966) by Stan Lee and Jack Kirby. His origin was later revealed in "The Silver Surfer" #1 (1968) by Stan Lee and John Buscema, which told how, as Norrin Radd, he convinced the world-devouring Galactus to spare his homeworld if Norrin would become his herald. He was then transformed into the space soaring Silver Surfer and helped Galactus find planets to feed upon.

When the Surfer broke free from his master, Galactus punished him by trapping him on Earth. During this time, the Surfer found himself growing more and more distant from humans and their violent ways. He eventually joined the Defenders with Dr Strange, Namor and the Hulk. The Surfer eventually escaped the Earth, at which point he became more involved with galactic events, joining the Annihilators and even becoming Galactus' herald again after the events of "Annihilation."

8 BLACK CAT

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The daughter of an international cat burglar, Felicia Hardy grew up and took cat burgling to a whole new level. First appearing in "The Amazing Spider-Man" #194 (1979) by Marv Wolfman and Keith Pollard, Black Cat has spent most of her career as a hero. At least, she sided with the heroes to get close to Spider-Man, who she dated on and off again before Peter settled down with Mary Jane. While her motivations were sometimes suspect, Black Cat mostly remained a hero over the years, regularly teaming up with Spider-Man (and helping herself to the odd bauble here and there).

During the original "Secret Wars" event, Black Cat visited Avengers Mansion looking for Spider-Man in "Peter Parker, the Spectacular Spider-Man" #90 (1984) by Al Milgrom. Due to the high number of missing heroes at the time, Vision offered Felicia a spot on the Avengers to fill in the ranks. She refused, however, because she was too fixated on searching for Spider-Man.

7 SILVER SABLE

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A mercenary from the fictional country of Symkaria, Silver Sable first appeared in "The Amazing Spider-Man" #265 (1985) by Tom Defalco and Ron Frenz. She was raised by her Nazi hunting father to be a fighter, and grew up to become the leader of the Wild Pack, a group of fellow mercenaries whose work helps fund Sable's home country. Their main goal was to hunt Nazis, although they'd take other work as needed. She and her team first encountered Spider-Man while chasing after the elderly thief Black Fox.

When Doctor Octopus brainwashed the Avengers during the "Ends of the Earth" storyline, Silver Sable helped Spider-Man fight against the Earth's Mightiest Heroes. After freeing the team, Sable and Spider-Man stormed Dr Octopus' lab in "Amazing Spider-Man" #687 (2012) by Dan Slott and Stefano Caselli. Unfortunately, Silver Sable was seemingly killed by the Rhino during this fight, so the Avengers never got to thank her with an offer of membership.

6 GAMBIT

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A thief from Louisiana, Gambit is a mutant with the ability to charge objects with kinetic energy, turning them into explosives.After helping Storm escape from the Shadow King in "Uncanny X-Men" #266 (1990) by Chris Claremont and Mike Collins, Gambit followed her back to the X-Mansion and eventually joined the team. Since his first appearance, Gambit has gone on to become one of the most recognizable and popular X-Men.

He's also one of the darkest and most mysterious. Before joining the team, Gambit was hired by Mr Sinister to participate in the mutant massacre of the Morlocks, although Gambit ultimately tried to prevent the slaughter. He was also suspected of being a traitor to the X-Men by Bishop, a time traveling mutant. On top of all that, Gambit was even turned into a horseman of Apocalypse for a brief period of time. With all of that going on, Gambit's had a hard enough time convincing everyone that he's not a villain, let alone even attempting to become of the Earth's Mightiest Heroes.

5 BLACK BOLT

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The leader of the Inhumans, Black Bolt's has a voice so powerful that just a single word can destroy cities. First appearing in "Fantastic Four" #45 (1965) by Stan Lee and Jack Kirby, the Inhumans are a reclusive group whose ancestors were experimented on and genetically modified by the alien race, the Kree. When exposed to the Terrigen Mists, one of the modified Inhumans develops a super power. Over the ages, the Inhumans remained hidden from the rest of humanity in the city of Attilan.

While Black Bolt has never officially joined the Avengers, he did join Iron Man's secret group the Illuminati, first appearing in "New Avengers" #7 (2005) by Brian Michael Bendis and Steve McNiven. The goal of this group was to gather the leaders of the various super human groups and try to prevent major catastrophes before anyone else saw them coming. While this group was heavily associated with the Avengers, it was never actually part of the Avengers lineup.

4 BETA RAY BILL

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One of the few characters that's worthy to lift Mjolnir, Beta Ray Bill is the member of an alien race called the Korbinites who were forced to flee their planet. With the entire race in suspended animation, the fleet approached Earth and was greeted by Thor in "Thor" #337 (1983) by Walt Simonson. Thinking that he was a threat, Bill fought Thor and separated him from his hammer. When Bill lifted Mjolnir, he was granted the power of Thor. Odin intervened and gave Bill his own weapon, called Stormbreaker.

Obviously, as an alien, Bill spends a lot of his time away from Earth. He did briefly join "Omega Flight" in 2007 during the aftermath of Marvel's Civil War, but he ultimately returned to the cosmos. He also joined the Annihilators, a team of massively powerful cosmic heroes who came together to protect the galaxy during a period where the Guardians of the Galaxy weren't active. Beta Ray Bill has joined Thor in Asgard many times, but he's never joined him as an Avenger.

3 HOWARD THE DUCK

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Even in the colorful and weird world of Marvel Comics, Howard the Duck is an odd character. First appearing in "Adventure Into Fear" #19 (1973) by Steve Gerber and Val Mayerik, Howard is actually from the planet Duckworld, but was brought to Earth when a demonic entity causes a dimensional rift during a battle with Man-Thing. He ended up in Cleveland, Ohio, where he encountered bizarre characters like Bessie, the Hellcow and Doctor Bong.

Howard has teamed up with many heroes over the years, including Spider-Man, She Hulk, the Defenders and Generation X. During the original Civil War, Howard attempted to register under the Superhuman Registration Act in "Civil War: Choosing Sides" #1 by Ty Templeton and Roger Langridge. This would have technically made him part of the Initiative, but it turns out that S.H.I.E.L.D.'s official policy on Howard is that he doesn't exist. This left him unable to register, but also absolved him of other responsibilities (like jury duty and paying taxes).

2 SPIDER-GWEN

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When high school student Gwen Stacy was bitten by a radioactive spider, she was turned into the amazing Spider-Woman. Her friend, Peter Parker, was a huge fan of the new superhero and tried to create a formula to make himself special like she was. Unfortunately, it accidentally turned Peter into a lizard monster, who was killed during a fight with Spider-Woman. It seems that every Spider-Man or Woman has to learn about power and responsibility the hard way.

Spider-Gwen (as she's known to the mainstream Marvel Universe), was first introduced in "Edge of Spider-Verse" #2 (2014) by Jason Latour and Robbi Rodriguez. She then teamed up with Peter Parker, the Amazing Spider-Man, for the multiversal adventure "Spider-Verse" by Dan Slott. Since then, Spider-Gwen has been helping out the Web Warriors, a group of Spider-People from alternate realities who protect the multiversal web. Given how busy she is with that, Gwen hasn't had time to join the Avengers in reality.

1 GHOST RIDER

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Johnny Blaze sold his soul to the devil (really Mephisto pretending to be the devil) to save his adopted father, Crash Simpson, from cancer in "Marvel Spotlight" #5 (1972) by Roy Thomas, Gary Friedrich and Mike Ploog. Not surprisingly, the devil tricked Blaze, and Simpson died in a motorcycle crash (technically meaning that he didn't die from cancer); thereafter, Mephisto turned Blaze into the Ghost Rider, the spirit of vengeance. Blaze was bonded with a demon, and at night would transform into a flaming-skulled, motorcycle-riding hellspawn that seeks vengeance against sinners.

Ghost Rider has never been an official member of the Avengers, although he was part of the Avengers of the Supernatural, in "Uncanny Avengers Annual" #1 (2014) by Rick Remender and Paul Renaud. This "team," however, was actually a group of supernatural based heroes that were brainwashed by Mojo to star in his television shows. Mojo, being a villain, doesn't have the authority to form an Avengers team, or to induct new members. So while it's debatable, Ghost Rider has never been part of an official team's lineup, and remains free of Avengers membership.

Can you think of any other Marvel heroes who never made the Avengers cut? Let us know in the comments!

"Avengers: Infinity War" will hit theaters May 4, 2018.