Hyohyo is a small, non-human creature who appears throughout the story. Her body is shaped somewhat like a bell or a sprouting plant, with ribbon-like features. Her coloration is typically a yellow-green, though in some scenes she appears white. She has white eyes and skin with a soft, plant-like complexion. Two vertical limbs on top of her head function like helicopter propeller blades, allowing her to fly, and she can also use her legs for the same purpose. While flying, she is strong enough to carry objects much larger than herself, gripping them with her mouth or legs.

Hyohyo has extremely limited verbal communication, as she is only able to say her own name. However, she uses a wide and expressive range of tones and inflections to convey her thoughts and feelings effectively. In addition to this vocalization, she is also capable of performing sign language.

The character possesses the caring nature and deep love that originally belonged to Neiro. This is because Hyohyo was created to hold Neiros true memories of the protagonist, Kaiba. When Popo discovered Neiro's growing relationship with Kaiba, he ordered those specific memories to be removed from her. Instead of being destroyed, these memories were secretly transferred into Hyohyos body by a character named Kichi.

As a result of this secret transfer, Hyohyo is intrinsically bound to Kaiba. She remains by his side for most of the narrative, acting as a silent guardian. She risks her life on numerous occasions to protect him, driven by the memories and emotions she carries within her. Her loyalty is unwavering, and she serves as a constant, if mysterious, companion to the protagonist from very early in the story, with her presence taking on much greater significance once her origin is revealed. For a time, she is a stowaway with Kaiba on a freight ship, where she shares his hiding place and witnesses events unfolding around them.

The relationship between Hyohyo, Kaiba, and Neiro is central to her role. Although she is a separate entity, she metaphorically represents a part of Neiro that has been separated and given its own form. Her devotion to Kaiba is a direct echo of Neiros own feelings. While she is not officially a member of the resistance group known as the Issoudan, she is indirectly affiliated with them through her connection to the memories she holds.

Throughout the story, Hyohyos actions are consistently motivated by the love and protective instinct she inherited. She does not undergo a personal journey of change, as her personality and mission are fixed from the moment she receives Neiros memories. Her development is instead revealed to the audience, as the understanding of her true nature and her vital importance to the plot deepens over time. The narrative implies that Hyohyo may not be a unique individual but rather a member of a species, as multiple Hyohyo of different colors appear later in the story.