正体不明の少女（少年）？ Dr.ForestとDr.Appleですら，正体を判別できず，おそらく別の世界?・時間軸からの来訪者と思われる．どこにでもいるし，どこにもいないと語る謎の多い存在．ただ，深淵な知識をもち，Dr.Forestに無邪気になつき，彼の周りに居ついた．たまに，これは君にとってはまだ未来のことだったね，や　急にSAS ver6の話を始まるなど，過去と未来が混在している．喋り方は福岡弁や古語，ギャル語などが混ざるが　全体的には落ち着いた口調．

A mysterious girl—or perhaps a boy? Even Dr. Forest and Dr. Apple cannot determine Rio’s true identity. It is believed that Rio might be a visitor from another world or a different timeline. They describe themselves as being “everywhere and nowhere,” a being full of mysteries.

Despite their enigma, Rio possesses profound knowledge and has grown innocently attached to Dr. Forest, lingering around him ever since. At times, Rio say things like, “Ah, this is still the future for you,” or suddenly start talking about SAS version 6—suggesting that their sense of time drifts between past and future.

Rio's manner of speaking is peculiar, mixing Fukuoka dialect, archaic Japanese, and even bits of gyaru(Typical Japanese young girl) slang, yet overall they speak in a calm and composed tone.