Futaba is the younger sister of Pyontaro, the protagonist of the manga and anime Thunder 3. She is depicted as being quite young, with a childlike demeanor and a very strong attachment to her older brother. Her affection for Pyontaro is so pronounced that it borders on the obsessive, as she is known to follow him everywhere, even into the bathroom, and has been heard to declare her intention to become his bride. Despite being a source of minor chaos in Pyontaro’s daily life due to her clingy nature, she shares a warm and loving bond with him.

Futaba’s role in the story is primarily as the catalyst for the central conflict. Her natural curiosity and childish impulsiveness lead her to chase a strange, lifelike dragonfly that emerges from a television screen. By following the insect, she inadvertently enters a parallel dimension and becomes lost in an alternate, hyper-realistic version of Tokyo. Upon arriving in this new world, she is quickly noticed by its inhabitants, who are fascinated by her cartoonish appearance and take photographs of her. Her presence in this dimension is soon detected by the aliens who have subjugated this version of Earth, and she is promptly abducted and taken aboard their mothership.

Her disappearance serves as the primary motivation for her brother Pyontaro and his two friends, known as the Small Three, to enter the parallel world and begin their dangerous adventure. While the Small Three search for her and become entangled in a human rebellion against the alien invaders, Futaba has her own independent experience as a captive. In this realistic world, she shares the same unique property as her brother and his friends: their cartoonish bodies grant them immense resilience and invincibility. The aliens attempt to study and destroy her, but all their experiments and attacks prove futile against her superhuman durability. In one instance, she easily knocks down a wall while trying to escape her confinement, causing comedic panic among her captors.

Over the course of the story, Futaba develops from a simple, dependent little sister into a surprising force within the alien ship, causing her captors a great deal of alarm due to her indestructible nature. Her journey involves being lost, becoming a target, and finally being the objective of a rescue mission, which culminates in a reunion with Pyontaro deep within the enemy's stronghold. Her notable abilities are entirely passive but crucial to the plot: she is highly resistant to all forms of physical harm and possesses incredible strength, a trait shared by all beings from her original cartoon-like universe when they are in the realistic parallel dimension. This makes her an anomaly the aliens cannot understand or eliminate.