Risa Momioka attends Sainan High School, a classmate of Rito Yuuki and Lala Satalin Deviluke. She begins as a 15-year-old first year, progressing to a 16-year-old second year. She stands 167 cm tall, weighs 54.5 kg, and has body measurements of B83-W60-H85, with light brown hair and brown eyes. She commonly wears the school uniform, often without a tie, and sometimes dons it on non-school days as a fashion statement.

Risa exhibits a fun-loving, mischievous, and overtly perverted personality. She frequently engages in sexually suggestive behavior, groping female classmates' breasts and whispering provocative comments. This conduct targets various girls, including close friends Haruna Sairenji and Yui Kotegawa, as well as extraterrestrials like Lala and Momo Belia Deviluke. Alongside friend Mio Sawada, Risa initiates or revels in ecchi situations and often gives misleading romantic advice to Lala about Rito. Her actions sometimes stem from underlying loneliness due to frequently absent parents, a feeling she confided in Rito about feeling isolated at night.

Her relationships vary. With Rito Yuuki, she maintains a teasing rapport, exploiting his shyness by creating sexually awkward situations or discussing intimate topics. Initially viewing him as unremarkable, she later acknowledges finding him "cute" when flustered and potentially harbors ambiguous feelings—exemplified when she affectionately called him "darling" during an interrupted intimate moment under Nemesis's control. With female characters, her interactions are consistently hands-on; she frequently gropes Haruna and Yui, overwhelms Momo in baths by fondling her and biting her ear, and provokes reactions by lifting Yami's skirt. Momo acknowledges Risa's advanced "technique" during these encounters, finding herself unexpectedly vulnerable despite her own tendencies.

Across the series, her development includes recurring comedic roles and subtle emotional layers. She debuts in the original *To LOVE-Ru* manga (Chapter 6) and anime (Episode 4) as Haruna's best friend with established groping habits. In *Motto To LOVE-Ru* (Episode 10), her teasing of Rito intensifies alongside her confession of nightly loneliness. *To LOVE-Ru Darkness* expands her role: Chapter 40.5 and the Darkness OVA depict her aggressively seducing Momo in a bath scene, culminating in a near-kiss before Momo flees. In Chapter 69, her interrupted encounter with Nemesis-controlled Rito hints at possible romantic feelings, with her subsequent lack of anger suggesting conflicted emotions. These moments show a pattern where overt sexuality masks deeper emotional needs, though her primary narrative function remains comedic provocation.