Hikari Myodo is Akina Myodo's younger sister. Ten years before the main events, she wandered into ruins during summer camp and fell prey to Sybilt. This possession cursed her with a chronic illness requiring frequent hospital stays. Despite her health struggles, Hikari projected a cheerful, bubbly exterior. She often played Cardfight!! Vanguard during hospitalizations, expressing admiration for professional players. Internally, however, she harbored deep guilt over how her illness impacted Akina, believing it stole both their childhoods. She specifically regretted the summer camp incident and wished she could undo it.

In the original timeline, Akina entered the Fated Clash tournament to wish for Hikari's recovery. He lost to Suo Yobitsugi, resulting in his death. Three years later, a traumatized and hardened Hikari, known as "Fox Mask," entered the next Fated Clash. She won and used her wish to travel back in time, forcing Akina to forfeit the original tournament to save his life. This act created a new timeline where Akina survived, won the Fated Clash, and cured the present Hikari's illness. However, erasing her original timeline caused Future Hikari to slowly fade from existence. She later confronted her present self to warn against further time travel, acknowledging her own past mistakes while demonstrating greater maturity despite their shared self-critical nature.

In this new timeline, after being cured, the present Hikari became corrupted by Sybilt. This corruption amplified her existing guilt and regret, turning her bitter and cold towards Akina. She resolved to travel back in time to prevent the summer camp incident, mirroring her future self's earlier motivation. Sybilt's influence linked Hikari to the entity Liael, who became her Avatar unit in Vanguard. Liael manifested in two forms reflecting Hikari's different states: Liael=Amorta (Keter Sanctuary), symbolizing Future Hikari's desire to rewrite the Fated Clash's tragic outcome through time-rewinding abilities; and Liael=Odium (Dark States), representing present Hikari's accumulated guilt and drive to undo the childhood incident via a power-storing mechanic enabling extra turns.

Hikari's internal conflict peaked during a confrontation between her future and present selves. Both versions clashed over their choices while acknowledging their shared regrets and self-blame for causing trouble. Future Hikari's intervention and eventual fading highlighted the dangers of time manipulation, a power resonating with Liael's dual existence across time. Following these events, Hikari's story continued as she engaged in school activities and friendly Vanguard matches with Akina, including a school festival duel where they wagered on meal choices.