The future of the Marvel Cinematic Universe has been made a little more concrete, with Kevin Feige confirming that Jack Kirby's Eternals have begun making their way to the big screen. Though things are in the earliest of stages, it doesn't hurt to familiarize with these cosmic characters -- particularly Sersi, who is believed to be the protagonist at the heart of it all.
Created by Jack Kirby in 1976, the Eternals are humans who were experimented on millennia ago by the Celestials, cosmic beings whose presence has been seen in the MCU already, per Ego in Guardians of the Galaxy Vol. 2. Thanks to the experimentation, the Eternals are long-lived, have accelerated healing, and have the ability to use cosmic energy in a variety of ways. Any trio of Eternals can also form together into the Uni-Mind, a powerful psionic entity with all the powers and abilities of the trio that comprise it.
Sersi is a fourth generation Eternal, and different from the rest in that she wants to live among the humans. In fact, more so than any other member of her kind -- save the Forgotten One -- she's lived a human life. First showing up in the group's titular third issue, she's tied primarily to Greek myth. When she was in ancient Greece, she met Homer, who would later write The Odyssey's character Circe based on her. During her time in Greece, she also imprisoned the imps within Pandora's Box; she was also present during King Arthur's court and Nero's Rome. All throughout her life, she switched from various occupations; from singer to actress, stage magician, and dancer, all while living her life as an adventurer.
Eventually, Sersi got to interact with members of the Avengers before she was officially asked to join the team. She fought alongside Thor in the Viking Siege of Paris, and a time-traveling Captain America met her as a young child (though at the time, she was actually thousands of years old). She wound up enjoying being a superhero more than she expected, becoming one of the Avengers' heaviest hitters at the time.
While she was an Avenger, Sersi found herself in a relationship with Dane Whitman, the Black Knight. But it wasn't so simple: the version of Dane she fell in love with was an alternate reality version of the Proctor, a man who was rejected by the Sersi of his universe who set out to murder every iteration of her across the multiverse. Neither Sersi or Dane were aware of that fact, or that Proctor was responsible for her continuing instability, brought on when she was forced to form the UniiMind with two other Eternals.
In her paranoia, she destroyed both the Avengers Tower and Brooklyn Bridge. Things got very strange for a time -- Proctor framed her for murder and wanted to use the life energy of her and a Watcher he killed to collapse all realities on one another -- but her fellow Eternals came to her aid by saving the Avengers and deducing that Proctor was responsible for everything, allowing her to eventually kill him with his own sword. Aghast at everything she did, Sersi entered a door between realities so she could live outside of his orbit, Black Knight in tow because he felt responsible in his own way.
Sersi's big return to the Marvel Universe came when Neil Gaiman took over writing duties for the Eternals in a 2006 miniseries, a story that could form some of the backbone for the Eternals film. Working as a party planner, she didn't remember being an Eternal or having powers, her memories flooding back when she accidentally turned a cat into a dragon. Thanks to Gaiman, using Sprite the Eternal as his mouthpiece, the cosmic history of the Eternals had been retconned as a very complex illusion, and Sersi is more like half a million years old. By the end of the issue, she's decided that it's just easier to live a normal life: she doesn't want to be an Eternal, and definitely not as an Avenger, as this was during the time of the original Civil War event where superheroes had to be registered. When last we saw her, it was in 2013's Avengers Assemble where she briefly helped the Wasp find Yon-Rogg, a Kree solidier.
It's easy to see why an Eternals film would happen with her as the lead; a powerful, ageless woman who's lived throughout history hits upon a number the same notes as DC's Wonder Woman. The Eternals' connection to the Celestials also allows the MCU to be even more cosmic than before, especially when taking her trips to other universes into account.