Io is one of the main characters from the fifth Keroro movie and is the brother of Rana. He is a Mana Pekoponian, a type of spirit that can take human form, and his age is estimated to be between eleven and fourteen in human years. Io is a protector of Roanoke Island, a role that defines his existence and daily activities. His birth date is unknown, though he is said to be as old as the moai statues, suggesting an ancient origin for his kind.

In terms of personality, Io displays a mischievous and playful side, first appearing alongside his sister to tease Fuyuki before revealing their true nature. Despite this initial trickery, he is fundamentally serious about his duties and shows genuine care for the island he protects. He is respectful toward the spirits called Mana, whom the islanders love, and he shares this appreciation. Io can also be expressive in his admiration, as shown when he claps approvingly after Keroro gives a speech, calling him very admirable.

Io's motivations center on safeguarding Roanoke Island and the Mana spirits that inhabit it. He explains to Keroro and Fuyuki what the spirits are, providing essential exposition about the nature of their world. Io and his sister confess that they themselves are Mana, demonstrating their deep connection to the spiritual ecosystem of the island. When a moai statue becomes a point of concern, Io and Rana warn Fuyuki about the dangers of removing it, specifically mentioning that a malevolent force named Aku-Aku will find them if the moai is not returned. This warning highlights Io's role as a guardian who understands the balance between the physical and spiritual worlds.

Key relationships in Io's life include his sister Rana, with whom he works closely as a pair of protectors. They appear together, act together, and share the same secret identity as Mana in human form. Io also develops a connection with Keroro and Fuyuki, though his perception of Keroro is unique. Both Io and Rana refer to Keroro as Kainini, a term that means gourmet, much to Keroro's displeasure. This nickname indicates that Io initially views the alien frog primarily as a potential food source, showing a mixture of innocent ignorance and otherworldly custom. Io is confused when Fuyuki calls the character by his proper name, Keroro, revealing that the siblings have their own naming system for the beings they encounter.

The story shows Io's development through his interactions with the main characters. He begins as a mysterious figure who faints Fuyuki and attempts to roast Keroro over a fire. After Fuyuki obtains a translator, Io is able to properly introduce himself, shifting from a potential antagonist to an informative guide. His willingness to explain the nature of Mana and warn the group about Aku-Aku demonstrates a cooperative turn. By the end of the narrative, Io shows respect for Keroro's speech, indicating an evolving relationship beyond mere confusion or culinary interest.

Io possesses several notable abilities. As a Mana in human form, he has the power to disappear at will, a trick he and Rana perform after confessing their true nature to Keroro and Fuyuki. This ability reflects his spiritual constitution and his connection to the invisible world of Mana. He also has knowledge of the island's history and dangers, including awareness of Aku-Aku and the significance of the moai statues. His physical abilities include catching Fuyuki after the boy faints, suggesting quick reflexes and strength.