Ruu is an alien baby from the Planet Otto, who can levitate and perform telekinesis on objects, and who develops a deep love for Miyu and Kanata, likening them to his parents and often assisting and helping them. He uses his abilities at very inappropriate times, causing Miyu and Kanata great trouble.
Although he is only a baby, he can understand everyone perfectly. Ruu loved Miyu and Kanata as much as he does for his own parents, and doesn't like it when Miyu and Kanata fight, like in the episode "With Ruu-kun at The Department Store".
in that episode, Ruu manages to make peace between his two beloved 'earth' parents. Ruu is also shown as an ordinary baby that cries and laughs like normal babies do.
In order to keep the others from knowing the truth about Ruu's identity, he is referred to as Miyu's little brother although Ruu will deny that and insist on calling Miyu "mama". So he believes Miyu and Kanata are his real parents but does not learn the truth until the end of the series.
In Shin Daa! Daa! Daa!, he met with a girl and fell in love with her. That girl is Miu, Kanata and Miyu's daughter. Ruu and Miu were apart after Ruu send Miu back to Earth, but they met with each other again when Miu was an 8 grade student.