WARNING: The following contains spoilers for One Piece Chapter #1021, "Demonio," by Eiichiro Oda, Stephen Paul, and Vanessa Satone, available in English via Manga Plus and Viz Media.
Eiichiro Oda's One Piece is highly acclaimed for its very detailed and extensive storytelling. The characters, especially the Straw Hat Pirates, are well fleshed out. Each one of them has a unique, tear-jerking backstory, giving their characters a distinct personality. Franky, for instance, focused on reconstructing himself and inventing various equipment after getting hit by the sea train. Similarly, Nico Robin's tragic past attached a deeper meaning to the word devil to her.
Robin was one of the most mysterious characters during the earlier arcs of the series. Unlike the other members of the Straw Hat Pirates, she asked Luffy to join the crew instead of being invited. She also started as one of the main antagonists and only turned a new leaf after being defeated. However, that isn't the reason she's referred to as a devil.
Robin's association with the word devil dates back to her childhood. She hails from the island of Ohara, a country mostly consisting of archeologists. As seekers of knowledge, the villagers delved into research on a daily basis. They didn't let anyone or anything obstruct them from learning the true history of the world, not even the World Government. The young Robin also had a penchant for learning, and it only heightened due to her Devil Fruit powers. She already ate her Devil Fruit before she was eight. It's also the reason why she's ostracized by both her family and other villagers, often calling her a devil.
The moment Robin mastered learning the language used in making the poneglyphs, Nico Olivia, her mother, returned to Ohara. The Marines were right on her tail and obliterated the entire island among its residents shortly after. Robin was the sole survivor at the expense of Jaguar D. Saul. Being a remnant of Ohara and being able to read the poneglyphs, the World Government hunted her and labeled her the Demon of Ohara. They falsely accused her of searching for Ancient Weapons in a bid to destroy the world.
Robin's Devil Fruit allows her to spring any body part in various locations. One of her skills manifests giant wings made from conjoined arms. While she can't fly with said wings, she can easily break her fall with it, easily giving the same image as a fallen angel. However, a more direct allusion to her devil sobriquet is the new technique she used against Black Maria, the Demonio Fleur.
During a cutthroat fight with one of Kaido's Tobi Roppo, Robin was forced to unleash all her trump cards. She channeled everything she learned during her stay with the Revolutionary Army in a technique called Demonio Fleur, a nod to her moniker the Devil Child. When activated, Robin manifests a giant version of herself with multiple appendages, fangs, and horns. She used her numerous limbs to lock her opponent in a hold and perform a clutch.
Nico Robin has one of the most eventful backstories among the Straw Hat Pirates. She has been associated with the devil since she was young, and it haunted her even after joining the Straw Hats. But just like what Zoro did in recent chapters of One Piece, she embraced her terrifying epithet for the sake of her Nakama.