Kotetsu, a katana tsugumomo bonded to Sunao Sumeragi, once served her late brother Suou. His name—merging "tiger" (虎) and "iron" (鉄)—mirrors his unyielding combat role. Sunao initially dismissed him as a tool, holding him responsible for Suou’s death during a clash with a formidable amasogi. Her resentment fueled reckless decisions, prioritizing pride over strategy, such as ignoring Kotetsu’s warnings during her duel with Kazuya Kagami. Overexertion shattered Kotetsu’s hilt mid-battle, leaving Sunao vulnerable and defeated.

This defeat spurred Sunao’s transformation. She embraced Kotetsu as an equal partner, fostering trust that sharpened their synergy. His capabilities—storing energy for devastating strikes, unleashing Ground Slash and Air Slash techniques—aligned seamlessly with her honed spirit power. Their collaboration birthed advanced skills: the Shadow Sword technique, the rapid-slash Secret Sword Technique Hibakiri, and Ascension mode, a potent fusion state countered by temporary paralysis post-use.

Rooted in the Sumeragi legacy, Kotetsu’s past intertwines with Sunao’s quest for vengeance after Suou’s death. When Kanaka Kagami defeated the amasogi first, Sunao shifted her rivalry toward surpassing Kanaka. Kotetsu’s role intensified during the Mayoiga Revolt and Tsuzura Temple conflicts, testing their alliance against external foes and ideological rifts among tsukumogami. Their journey traces Sunao’s evolution from arrogance to humility, framed by mutual growth and respect.

Though Kotetsu’s explicit personality remains sparse in records, his steadfast presence and Sunao’s progression imply unwavering loyalty. His combat prowess and symbolic weight in the Sumeragi lineage cement his dual role as a legendary weapon and a narrative anchor exploring partnership and legacy.