Note: not a Lannister.
The Eternal with an affinity for humanity, Sersi has the ability to manipulate matter, changing the make-up of any non-sentient material she touches.
Sersi is easily the most famous of the Eternals, but primarily because people forget she is an Eternal. She was a member of the Avengers for a time - and not just some random off-shoot team, the real, main Avengers! She joined the team in 1990, and while the team she was with initially wasn't great, she was around through the first half of the decade (aka "the bomber jacket years"), working with powerhouses like Black Knight, Giant-Man, Vision, Black Widow, Hercules, Captain America, Thor, and Crystal. The inspiration for the sorceress Circe in The Odyssey, Sersi has the power of matter transmutation in addition to all the other "flying brick" powers all the Eternals have; there was no "non-sentient" clause in the comics, because how else would she turn Odyssesus' sailors into pigs?
There have been lots of actors who appeared in more than one comicbook role, but Gemma Chan is the first to have two major roles both in the MCU - remember, she was Captain Marvel's rival Minn-Erva just two years ago. There's a more recognizable likeness here, because her face isn't half-covered by a mask and jawline armor.
Ever since Kirby created her, Sersi's main costume has been basically a green swimsuit. It's what she was wearing in the '70s,
and it's what she was wearing two decades later when she joined the Avengers. In fact, her changing out of that costume for one in red and black was an indication of her deteriorating mental state (it wasn't her fault - she was being interdimensionally stalked by a pre-internet Nice Guy). The movie trades that for something a bit more practical, though it still retains the green. This is a yellower shade than Sprite's teal, but the metallic sheen works really nicely with the silver accents. She now gets to wear a minidress over leggings, a perfectly normal kind of outfit, and I bet it wouldn't take five minutes of looking around online to find somebody complaining that we can't see her cleavage.
Sersi has two loosely gesturing hands, one fist, and one with two fingers extended like she's just met Dr. Gräfenberg and is helping him with his research. She's also got the left leg of the Gilgamesh Build-A-Figure.
Despite being a '90s Avenger, and therefore (fairly) recognizable, Sersi has never had an action figure. This movie version may not look much like the comics, but it will be a great stand-in in a pinch.