Well there is an organization Medical Mechanica that wants to flatten every world to make it all the same. Then there is Haruko(sp)) (Ramaru sp?) and she is in love with Atomsk who is the king of all space pirates. He was apparently captured by medical mechanica but he escaped in the form of the robot (Kanti). Nauta (Sp?) turns out to be a medium for medical Mechanica and in the final episode of the show he is eaten by the terminal core which s going to flatten earth and he absorbes atomsk and destroyes medical mechanica. Then he goes back into space. Thats what I got out of the Anime
Naota learns to be more mature by not forcing himself to act like an adult. Haruko's comments about Naota just being a kid are saying that he should stop trying to act like an adult because he's just a kid. The final time she says it. She's talking about how he's finally realized it and has begun acting his age.