Hikaru Shidou, born in Tokyo, Japan, is the daughter of a kendo instructor father and a mother, with three older brothers named Satoru, Masaru, and Kakeru. During kindergarten, she defeated her father in kendo, prompting his permanent departure on a training journey. She cherishes her dog, Hikari. A junior high student standing 145 cm tall, she is often mistaken for an elementary school student. Her interests include biology, playing with Hikari, and kendo club activities, aspiring to become a seeing-eye dog trainer. She favors ice cream and chocolate, dislikes spicy food, and prefers the color red.

Hikaru possesses light red braided hair and fiery red eyes. Initially wearing a school uniform of a red blouse, black skirt, white ribbon, and red-trimmed petticoat, she later dons white ruby-adorned armor upon arriving in Cephiro. Her armor evolves into a short red, white, and gold dress with a tiara and cape.

Cheerful, energetic, and straightforward, Hikaru displays athleticism and excitable enthusiasm. Though youthful and occasionally childish, she exhibits bravery, combat maturity, and strong responsibility. Honest and empathetic, she prioritizes others' well-being but internalizes personal struggles. Trained in kendo since childhood, she wields exceptional swordsmanship and communicates with animals, including Cephiro's fauna like Mokona and Clef's pet, Fyula.

As the Magic Knight of Fire, Hikaru wields a sword forged from Escudo that ignites when touched by others. Her fire spells include "Flame Arrow," "Crimson Lightning," "Dance of Blaze," and "Meteorite of Flame." She pilots the fire Rune-God Mashin Rayearth, resembling a lion or wolf, which evolves with her resolve.

During a Tokyo Tower field trip, Hikaru is transported to Cephiro alongside Umi Ryuuzaki and Fuu Hououji, summoned by Princess Emeraude to defeat High Priest Zagato. Guided by Clef and Mokona, she embraces her role, secures her weapon, awakens Rayearth, and forges deep bonds with Umi and Fuu. After defeating Zagato, she learns Emeraude orchestrated her own death due to forbidden love for him. Hikaru reluctantly kills Emeraude, returning to Earth traumatized.

In the manga’s second arc, Hikaru becomes withdrawn, frequently meeting Umi and Fuu at Tokyo Tower. Wishing to return to Cephiro, the trio arrives to find it crumbling without a Pillar and under invasion. She befriends Zagato’s brother Lantis, apologizes for Zagato’s death, and questions the Pillar system’s morality. Chosen as a potential Pillar alongside Autozam’s Eagle Vision, she wins by refusing to fight him, asserting unnecessary deaths cause pain. As Pillar, she abolishes the system, distributing responsibility for Cephiro’s stability to all inhabitants.

In the anime’s second season, Hikaru battles guilt over Emeraude’s death and nightmares of Debonair. Captured by Autozam, she bonds with Eagle and loses her sword to Nova—a soul fragment born from her grief—reclaiming it with Sierra’s aid. She develops romantic feelings for Lantis, rescues him with Eagle’s help, and confronts Nova and Debonair. After Eagle’s death, she accepts Nova back. Post-Debonair’s defeat, she becomes Pillar, ends the system, and reveals she summoned herself and her friends back to Cephiro.

In the OVA, Hikaru attends school with Umi and Fuu. After encountering Mokona, she receives fire powers from Clef and pilots the renamed Mashin Lexus, battling Eagle Vision as he attempts to destroy Earth for Princess Emeraude. She prevails when Eagle’s sister intervenes.

Hikaru’s relationships define her journey: unwavering loyalty to Umi and Fuu cements their trio as inseparable; her romantic connection with Lantis is prominent in the anime, while the manga depicts ambiguous affections for Lantis and Eagle Vision. She extends empathy to antagonists like Ascot and Caldina, persuading them to reconsider their actions.