Mikoto Saitō, a high school student sharing classes with Akina Myodo, commands constant attention as a prominent figure at their school. She actively pursues a career as a talented singer and idol but suddenly announces a hiatus from entertainment, partly prompted by critical comments labeling her as fake. Her surname combines the kanji for "west" (西) and "tower" (塔).

Physically, she sports waist-length black hair and glasses. Her signature outfit features a long-sleeved white shirt with a folded collar layered over a black tank top, a pink pleated skirt secured by a brown belt, and white leggings.

Mikoto enjoys online matches of Cardfight!! Vanguard but had never handled physical cards until selected for the Fated Clash upon obtaining the "Fated One of Ever-changing" card. This designates her among the six "Fated Ones," individuals chosen by cards from Cray’s six Nations due to desires resonating with avatar units. After observing Akina Myodo’s victory against Nao Inabe, she discovers Akina was sent by his sister Hikari—a devoted fan. Akina introduces her to Megumi Okura, who assists in constructing her physical deck, leading Mikoto to win her debut match against Megumi.

Her Fated Clash journey sparks personal growth; she realizes the experience alone can fulfill her desired changes without requiring victory. For her first official match, she faces Taizo Kiyokura, previously encountered in Hikari’s hospital room during a visit with Akina. Taizo emerges victorious.

A decade before these events, Mikoto attended a Kiyokura Group summer camp alongside Akina, Hikari, Nao Inabe, Suo, and Kuon, with Taizo as counselor. There, she bonded closely with Hikari, though most participants later forgot these memories.

As an idol, Mikoto performs the insert song "Brand New!!" during pivotal moments, echoing her artistic roots.