Abandoned as a baby in a small Gallian village, Riela Marcellis was raised by an elderly couple with no memory of her biological parents. Following her adoptive mother's death shortly before the Second Europan War erupted, she joined the Gallian Militia, partly driven by feeling unwanted in her village due to her distinctive appearance. Her notable features include long, two-toned red hair with silver tips—often tied back with a black ribbon—and a well-endowed figure. She stands 162 cm tall. On duty, she wore the standard Nameless uniform: black jacket, collared white shirt, black corset, red skirt, belt pack, brown gloves, leggings in Nameless colors, and long brown boots. Off duty, she wore a pink two-piece swimsuit at the beach, and for her wedding, a strapless gown with veil, tiara, heart-shaped earrings, pearl necklace, and her hair styled in a bun emphasizing the silver portions.

Riela's early military career gained infamy. All five squads she joined within her first month suffered total casualties except her, earning her the nickname "Grim Reaper." This pattern stemmed from latent Valkyria healing abilities allowing her to survive fatal injuries that killed comrades. The resulting stigma caused severe ostracism, leading to her transfer to Squad 422 ("Nameless") to address morale issues. Initially treated as an outcast even there, her perseverance and dedication gradually earned acceptance. To disprove the "Grim Reaper" label and protect allies, she insisted on solo reconnaissance missions, demonstrating inherent kindness and compassion. She was self-conscious about weight gain and rapid breast growth, which she attributed to her love of milk and its calcium benefits. She also collected good luck charms, though she never found a four-leaf clover.

Her character development centered on embracing her Valkyria heritage. During a mission at Ghirlandaio, she encountered Clementia Förster, who gave her a ragnite relic that reacted with her body, triggering her first Valkyria transformation. Förster deemed her "defective," exacerbating Riela's fear her powers would harm allies. Nameless' leader, Kurt Irving, challenged the "Grim Reaper" superstition, arguing no evidence linked her to the squad fatalities. His support helped restore her confidence, and she eventually activated full Valkyria abilities to protect Nameless. In this form, her hair turned entirely silver, her eyes glowed red, and she radiated a blue aura while wielding ancient Valkyrur attire and weapons, granting temporary invincibility and enhanced combat capabilities.

Riela nurtured romantic feelings for Kurt, often taking an assertive role in their relationship despite his reserved nature. Her trust in his leadership was absolute, even when doubting his risky strategies. Post-war, they married in a ceremony attended by Nameless members, where she expressed profound happiness. In other media like *Project X Zone*, she appeared as a playable character alongside Kurt and Imca, using her Valkyria powers defensively and showcasing her cheerful, nurturing personality.