Ostensibly a middle school student at Fuuka Academy, Nagi is a mysterious waif-like boy who knows many secrets of the HiME Carnival. In addition, he exhibits various supernatural powers throughout the course of the series, including an ability to materialise or summon Orphans, cause articles to spontaneously combust, the ability to eavesdrop on conversations from a massive distance and a means to act as a conduit for the words of the Obsidian Lord, who he is revealed to serve.
Almost always seen in a high vantage point, Nagi's initial appearance takes place after Mai Tokiha's first meeting with Mashiro Kazahana and Fumi Himeno, giving them an update as to the HiME situation. Around this time, he enables Mai and Akane Higurashi to summon their Childs in order to fight Orphans, telling them that, when using Childs, they must place the "thing they value the most" on the line (which they presume to mean their own lives).
He is later taken aback by the machinations of the Searrs Foundation and their ability to create Orphans of their own, before opposing them during their invasion of the school. When this is defeated by Mai and her Child Kagutsuchi, Nagi's next appearance is to inform the HiMEs of the true implications of the HiME Carnival.
He is later seen serving the Obsidian Lord and, in this capacity, he opposes Miyu Greer, rebooted by Midori Sugiura, before she defeats him, allowing Suishohime to resurrect the fallen Most Important People and end the Carnival for good. Nagi is exorcised after the combined HiME forces defeat the Obsidian Lord, before he and Mashiro depart through a door, watched by Fumi.