Prior to the events of the series, Emilia Mikogami lived a relatively normal life as a Japanese high school girl, going by the name Himiko. Her life was upended when she was nominated to participate in the real-life version of the Btooom! game, a brutal death match where participants use specialized bombs (BIMs) to kill others and survive.

Before arriving on the island, Himiko had suffered multiple betrayals and traumas, particularly from people she once trusted, which led her to develop a deep mistrust of men. This emotional scar greatly affects her behavior when she first encounters Ryōta. Over time, however, she begins to open up and rely on him, becoming a key emotional and strategic partner in their struggle to escape the game alive.

Himiko is a teenage girl with long blonde hair, usually styled in twin tails, and vivid blue eyes. She wears a standard schoolgirl uniform, which becomes tattered and worn as the series progresses. Her appearance is often associated with fragility and innocence, yet her expressions and body language convey strength and alertness.

Her physical attractiveness often draws unwanted attention, but she proves herself capable and resilient, adapting quickly to the dangers of the island.

Himiko is initially cautious, mistrustful, and emotionally guarded, particularly around men. This stems from a traumatic betrayal prior to the game, where she was assaulted by individuals she once considered friends. Despite this, she is not cold or unkind merely cautious.

As she bonds with Ryōta, she reveals her compassionate, intelligent, and loyal side. She is resourceful in combat, capable of making quick decisions under pressure, and willing to put herself at risk for those she cares about. Her emotional arc involves reclaiming her trust in others, especially in Ryōta, with whom she eventually forms a deep romantic connection.