Princess Romelle is a female character of Voltron, which is based on the series Beast King Go-Lion. She is the love interest of Sven, who once was one of the Voltron Pilots, and to her misfortune she briefly was a prospect one for the wicked Prince Lotor.
Pollux, the planet that Romelle hails from, has been hostile to Princess Allura's Planet Arus due to convoluted history. Her father Coba allies himself with Planet Doom in hopes to defeat Voltron and conquer Arus, then backstab his Doom "allies". While his sons Avok and Bandor agree with him, his only daughter secretly longs for the day when both planets will be at peace.
When Lotor visits Arus, he's stunned when he sees Romelle and realizes how much she looks like Allura. Despite having been pleasant to her at first, he has her abducted as his prisoner of war after Pollux is taken over, the King is incapacitated, and Avok is first made into a Robeast and then goes missing.
As Lotor's prisoner, Romelle is subjected to lots of humiliations coming from him due to her position as the beautiful princess of an occupied planet and (to a smaller degree) a girl who resembles the woman he's obsessed with, including an escape attempt and then her recapture. This reaches a breaking point when Lotor uses Romelle twice as a bait in his plans to finish both Voltron and her still active younger brother Bandor, who had allied himself with Arus; this even included hanging her in chains in his spaceship. After this, Lotor drags Romelle to a place in Planet Doom named the Pit of Skulls and demands for her to marry him, which she hotly refuses.
As Romelle is thrown into the Pit, she finds a man who helps her escape and hide. The man is Sven, the former pilot of the Blue Lion, who was almost killed and then enslaved by Planet Doom's forces. Despite helping her, the man refuses to rejoin his friends due to how he sees him as deeply unfit for it, after spending months of enslavement and torture that have defiled him. Romelle, however, refuses to believe that he's a broken individual and tells him that he's still a hero and still can help the world. They reach an agreement and manage to escape together, then are reunited with their friends. Sven sees that Allura has become a good Voltron Pilot and decides to stay in the now liberated Pollux.
Some time later, Romelle and Sven gather some Polluxian warriors and decide to help Voltron directly. They form a resistence cell that greatly helps Voltron when it's time to take a direct action towards Planet Doom and its leadership, rather than fighting them off when they deploy their plans. They're both there when Doom is sieged by the Voltron team and other groups that have joined them, and then Sven is captured and almost dies to bring Lotor down, but he manages to live.
In the Western-exclusive final season of Voltron, Sven is now an ally of Pollux and its leaders Romelle and Bandor. Their feelings for one another have reached romantic stages but both are reluctant to pursue them, since Sven believes that he cannot love a princess like her and she's too reserved to fully speak about her love either. Eventually, an episode has the Royal Palace under attack and both siblings captured, and Romelle gets the chance to openly tell Sven that she loves him. Said episode finished with the two in an embrace.
In Voltron Force, an episode is dedicated to an ill Sven's struggles to rescue his son. The mother of the child is neither seen nor mentioned by name, but the producers have said that she is Romelle and that she was supposed to show up, but time restraints said otherwise.
In the original GoLion, Romelle is named Amue. Sven is actually the amalgamation of two characters: Takashi Shirogane, who dies early in the series, and his brother Ryou Shirogane. Takashi perishes before Amue is introduced so she only gets to interact with Ryou, who wants to fight the Galra Empire led by Prince Sincline (Lotor) as revenge for his brother's murder. In the end, he kills Sincline at the cost of his own life.
There's the implication that, when Amue was a prisoner of Sincline, she was sexually abused by him off-screen due to her similarities to Princess Fala (Allura). He is also seen slapping her to the ground, grabbing and dragging her by the hair, and trying to get her executed (instead of tossing her into the Pit of Skulls). The abuse as a whole is downplayed in Voltron, which implies non-sexual torture instead (though fans believe that she could've been raped as well) and cuts the instances where she's being beaten.