Luu 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. Luu 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 Luu-kun at The Department Store".
in that episode, Luu manages to make peace between his two beloved 'earth' parents. Luu 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 Luu's identity, he is referred to as Miyu's little brother although Luu 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. Luu and Miu were apart after Luu send Miu back to Earth, but they met with each other again when Miu was an 8 grade student.