Houki Shinonono, a central figure, originates as Ichika Orimura's childhood friend. Their bond formed when Ichika defended her from bullies, creating closeness despite her difficulty expressing emotions. Raised in a shrine and kendo dojo, she developed a disciplined yet reserved personality, with kendo becoming her emotional outlet and link to Ichika after their separation. A pivotal childhood promise occurred: Ichika vowed to become her boyfriend if she won a national kendo tournament. However, on the tournament day, her sister Tabane Shinonono's global unveiling of Infinite Stratos (IS) technology forced Houki's family into government protection, causing her withdrawal and lasting resentment toward Tabane.

Following the separation, Houki continued rigorous kendo training, winning a national tournament the year before entering IS Academy. During this period, Tabane provided the experimental IS unit Akatsuki. Overwhelmed by its power and seeking validation, Houki challenged legendary pilot Chifuyu Orimura, inflicting critical damage that forced Chifuyu's retirement. Consumed by guilt, she had these memories suppressed by Tabane.

At IS Academy, Houki reunites with Ichika but initially maintains a cold demeanor, enforcing formal address and displaying jealousy toward other female students. Her early academy life reveals emotional instability; she frequently resorts to violence, struggles with hypocrisy regarding her sister's legacy, and possesses low self-confidence regarding femininity. This manifests in discomfort wearing revealing clothing and reluctance to leverage her appearance for Ichika's attention despite occasional considerations. Academically, she starts as a C-Rank pilot with limited technical knowledge, initially using the training unit Uchigane.

Character development accelerates when Tabane gifts her the fourth-generation IS Akatsubaki. This unit, equipped with twin katanas and the energy-transfer ability "Kenran Butō," amplifies her kendo prowess but also magnifies her aggressive tendencies and belief in "might makes right." Her insecurities intensify upon witnessing Ichika's interactions, culminating in the Akatsubaki Incident where the IS temporarily takes control, attacking friends to fulfill her subconscious desire to possess Ichika. Confronting her past actions against Chifuyu and accepting Ichika's forgiveness, she resolves to address her emotional weaknesses rather than escape them. This leads to the loss of both Akatsubaki and Akatsuki, returning her to ordinary student status without resentment.

Her romantic feelings for Ichika remain constant, expressed through indirect actions like cooking or encouraging his kendo training rather than overt advances. Her envisioned future, revealed during the "World Purge" scenario, involves cohabitating at her family shrine as equals in kendo skill. Physically, she has long dark brown hair, typically in a ponytail secured by a ribbon (later replaced by one from Ichika), dark blue eyes, and a curvaceous figure. Her attire evolves from the standard IS Academy uniform modified with white stockings and boots to a custom white-and-black IS training suit after receiving Akatsubaki.

In the "Long Vacation Edition" OVA, which extends the first episode of the second season, Houki participates in a pool outing during summer break. While focusing on group interactions and an unrelated IS theft incident, it reinforces her established traits: discomfort wearing swimwear and persistent yet restrained romantic tension around Ichika within the group dynamic. Her journey concludes with her continuing her studies at the academy, focused on personal growth and maintaining complex but improving peer relationships without her personal IS units.