Rena Mouri is a 17-year-old Tokyo high school student with chest-length brown hair and brown eyes. Born on June 7th, she is a Gemini. Her personality is cheerful and lively, aligning with the fashion and sociability-focused "gyaru" subculture. Rena is a significant influencer on the social media platform "Kawasta" (or "Kyawasta"), boasting tens of thousands of followers, especially popular with youth.

She is the childhood friend of Ryuuhei Oda. They accidentally entered an alternate reality, encountering a grotesque creature before being rescued by Tris, a being knowledgeable about this other world. Tris granted Rena and Ryuuhei power, choosing them as "Knocker Ups"—individuals tasked with combating creatures known as "Weirds" or "Desaria" that emerge from the dream-state dimension Traumerei. Rena officially became a Knocker Up in the anime's third episode.

Fighting alongside Ryuuhei and more experienced slayers like Aruto Fushibe and Jessica Clayborn, Rena's combat abilities are demonstrated in a notable arc where she executes a climactic sniper shot during a confrontation. While specific developmental details are limited, her personal journey receives significant narrative focus within the series. Rena is identified as a main character and secondary protagonist within the story's framework.