This is about the anime characters. For the monster itself, see Astro.
The Astros and Captain Astro of the anime series have a sad history with humans, it seems. After they accidentally crash-landed on Pangaea, humans ensalved them. At some point after that, Durahan and his troops took over the village that they lived in and imprisoned the Astros to use for himself.
Durahan eventually releases them to kill the Searchers, but during a battle in the Madella Valley Genki has to risk his life to save Captain Astro, and the already weary captain realizes that killing humans will not fix the problems in their hearts. As he lays dying, he tells his troops to go out into the world and make peace.