Hikari Momoyama, younger sister of Mirai Momoyama, shares her family's indigo eyes with a slightly darker tint and peach-colored hair styled in short curled pigtails secured by pale orange bows. Her straight-cut bangs gather at her forehead's center, with thin strands framing each ear. From Season 2 onward, her pigtails lengthen and incorporate ribbons, while her forelock strands develop a slight curl.

Hikari displays maturity beyond her age yet often expresses bluntness, particularly toward Mirai, frequently doubting or questioning her sister's actions. Despite this critical exterior, she harbors underlying affection and support for her family, though struggles to convey these feelings openly. She voices opinions directly, even toward her parents, and is easily annoyed. Her behavior reflects typical childhood traits: she engages in play, enjoys drawing, requires regular naps, tires easily, and assists at the family shop.

Her relationship with Mirai involves frequent teasing, yet she cares deeply for her sister. She interacts closely with classmates Devi Fudo and Luu Asuka, who affectionately call her "Little Yama" (妹山), referencing her status as the Momoyama family's youngest member. In Episode 132, she assists them in preparing for their first performance to aid the ill Bugcchu. This collaboration strengthens their bond through shared effort and practice, evolving them into closer friends.

Her name "Momoyama" (桃山) combines "peach" (桃) and "mountain" (山), while "Hikari" (ひかり) translates to "light." Initial plans included her performing with the group Oshama Tricks, but this was abandoned due to production constraints.