These two antagonists are interesting, because they seem to be the only overtly villainous green eaters in the series. Or at least they can be interpreted that way – I can’t recall if they ate anything on screen but I assumed any speaking flyer characters were whacked with the “eats green food” stick.
Anyway, they aren’t aligned against the gang because of the necessity of their diet, but out of selfishness and greed. I’ve become a bit of a fan of villains who’re among the protagonists’ own class/species/group. That makes them more threatening and relatable. Rinkus appearing weak and silly while being quite scheming and nasty is an example, as I can picture people like that existing in real life. Sierra can also be deceiving, since I recall encountering other characters like him who’re far more heroic, but then you’d realize how nasty he can go.
I do like the thought of them eventually forming a sort of friendship later on. Like Sierra saves Rinkus and dusts him off, saying losing him would make accomplishing goals more troublesome and Rinkus is uncomfortable but intrigued, knowing what is being implied. I’m fond of the thought of antagonists who’ll never think of joining the side of the protagonists but have genuine connections and loyalty to those among their group.
I kind of wonder how these two fell in with Pterano. Did Pterano persuade them to go with his ambitions? Did Sierra and Rinkus do the persuading? Were they the only ones Pterano could find that would agree to go along with his plans? However it happened, it would probably influence what might happen if the pair ever bump into Pterano again. Those two will probably know just what to say to get under his skin and the conflict can be delicious.