Rebecca Anderson, nicknamed Becky, is a 13-year-old student at Kinokawa International School in Japan. Cinque Izumi's close friend from Earth, she exhibits a busybody attitude and understands his personality deeply. She visits Cinque's house every morning to wake him for school and shares a longstanding promise with him for their families to attend flower viewing during spring break. Her interests include fashion, video games, manga, and fantasy-themed novels.

Rebecca has light brown hair typically styled in two pigtails with ribbons and green eyes, often noted for her cute and pretty demeanor. Initially living an ordinary Earth life unaware of Cinque's adventures in Flonyard, her role expands significantly when Cinque brings her there in the second season. Princess Couvert Eschenbach Pastillage selects her as the Hero of Pastillage, granting her the holy sword Mercurius. This weapon adapts to her combat style, transforming into a flying broomstick or a hand cannon. Her magic-based attacks center on "Explosion Shot," utilizing a deck of magical cards for offensive spells and aerial bombardment. Despite lacking athletic prowess, her gaming reflexes enable effective dodging and precision strikes.

Rebecca later receives a Demon Crystal from Valério, enabling a temporary transformation into an adult form with enhanced magical power, featuring a bridal-esque costume with a veil. Her combat style evolves into long-range magic, including wave-motion guns, leveraging her strategic mind honed from bullet-hell games.

Emotionally, Rebecca harbors romantic feelings for Cinque. Early interactions show tsundere tendencies, but she matures in acknowledging his relationships with others, particularly Millhiore. By the second season's conclusion, she confesses her love to Cinque, who reciprocates platonically. A significant development occurs when Cinque attempts to address her as Rebecca, but she requests he continue using "Becky," indicating unresolved emotional complexity.

Her relationships extend to Flonyard's inhabitants: she becomes a cool big sister figure to Couvert and forms a close friendship with Millhiore, sharing candid discussions about Cinque. Initially shy around strangers, she gradually finds belonging in Flonyard's community. Her journey highlights growth from a reserved girl to a confident hero balancing Earthly connections with interdimensional responsibilities.