Kotona Tsutsumi is a twenty-four-year-old woman acting as the Guardian of Katsushige Shibata, one of the candidates to become the next head of the renowned Yotsuba family. She is a second-generation modified body created under the designation Bard Series, a lineage designed for exceptional affinity with sound-based magic. In the Yotsuba Succession Arc, she serves alongside her younger brother Kanata Tsutsumi, who is also a Guardian to Katsushige. Kotona is romantically involved with Katsushige and they are already in a common-law relationship; however, she has resigned herself to never entering a formal engagement because the unstable nature of her artificially modified genes precludes it. This personal conflict underpins her motivation: she is wholly devoted to Katsushige as both a protector and a partner, yet her very existence as a modified body creates an insurmountable barrier to the life she might wish for.

Her magical specialty lies in sound-based phenomena. As a Bard Series construct, she possesses a high aptitude for manipulating sonic waves, enabling her to execute precise acoustic attacks, scan environments through reflected inaudible high-frequency sound, and bolster her own actions with sound information reinforcement. In the film, she and Kanata confront the overwhelmingly powerful Tatsuya Shiba in an attempt to fulfill their duty, only to be decisively outmatched. The encounter demonstrates her capability and commitment, but it also underscores her limits against a truly exceptional opponent. Throughout the arc, Kotona remains composed and professional, yet her underlying vulnerability surfaces whenever the topic of her genetic inheritance or her future with Katsushige arises. Her development is subtle, showing an acceptance of her own nature and an unyielding loyalty to her master, even while she quietly carries the personal cost of her engineered existence.