This post may contain spoilers. Read at your own risk.
Shuvi was the first Ex-machina to be introduced in Tet‘s story which he was telling Izuna. In the story she seeks to understand the human [heart] and was found by Riku.
She looks like a young human but with several differences, being two tails coming out from her back and other machine like features protruding from around her body. She bears many similarities with Shiro. During her time with humans other than Riku, she wore a robe to cover the mechanical parts on her body, including the piece on her head. However, the tail was uncoverable (as it was her power source) Riku decided to go calling it equipment.
Her behavior is similar to that of a stereotypical artificial intelligence. She can absorb and process data and intelligence very quickly and act in the appropriate manner, a unique characteristic of her race. However, she seems to have difficulty in comprehending emotions and desires, otherwise characterized by her as the [heart]. Over time spent with Riku, she seems to have started to develop her own [heart]. Her expressions and feelings became more vivid over time.
After setting her personality to mimic that of a “shy Imanity survivor” and spending a year around Riku’s colony, she is unable to reset her personality back to the default monotonous mannerisms of the Ex-machina, stating that she is “stuck” in that mode.
Riku was an 18 years old Imanity who lived during the Great War on Disboard. He was one of the main protagonists in Tet‘s story.
He encountered Shuvi while going on an expedition and brought her to his colony after having lost in a game against her (albeit willingly, as he judged that winning was impossible, so instead he changed the bets to his favor).
He bears many similarities with Sora, being his hairstyle, slender body, black eyes and his face.
Ever since his home was destroyed and he took over as leader of his colony, he’s had a somewhat split personality. When the figurative lock on his [heart] is locked, he appears emotionless, cold, and calculating. When his heart is unlocked, the emotions he bottled up are released (often violently). His endeavors with Shuvi has since lessened his split personality and he appears to be more open around her.
(Source: No Game No Life Wikia)
So, the movie will actually star these characters, Riku and Shuvi. Though, they may seem to look a lot like Sora and Shiro, they are actually diferrent characters from Sora and Shiro.
The movie will adapt volume 6 titled It Seems the Gamer Couple Challenged the World, it is a prequel story that covers the Great War between the Gods. The story is a little darker and me reading some articles and wikias, I got myself spoiled.
I don’t actually know these characters and I have no idea of their background story. But, I’m kind of liking these characters just by looking through their appearances. They resemble Sora and Shiro by appearance and Riku and Shivu are actually couples. So far, I’m looking forward for the movie and I’m waiting to see how these new characters are. But, I’m guessing that I’m going to love the new characters even though I don’t know much of their details.