This is about the anime characters. For the monster itself, see Weed.
The Weeds permeate the anime series. A troupe of them live in Moo's floating castle as his attendants, and several of them are in charge of piloting his iron Birds. Captain Weed seems to be in charge of the fleet and the castle's artillery capabilities. When Moo begins to make the transfer into his old body, he relies heavily on the Weeds to run the machinery (including the Final Gate, which they learn how to operate when they capture and torture Monol for information).
Captain Weed is Moo's personal emissary to the Big Bad Four, and works extensively to coordinate efforts between Durahan and Moo. He is actually able to steal the Magic Stone from the Searchers, but when he comes to the realization that Durahan is planning to take the stone for himself he defends it for Moo, giving his life for his master when Lilim finally murders him.
At the end of Season 2, Moo turns on his own troops in a fit of rage and destroys not only his floating castle but his entire fleet as well, including all of his trusty Weed followers....except one.
In Season 3 it is revealed that his Weed fleet commander survived the crash of their ship, thanks to some of the same cybernetic enhancements that have kept Durahan alive. He nows serves under the evil warrior, trying to recapture Moo's Dark Disc to exact their revenge on the monster who tried to destroy them all.