Darius was always meant to be the leader of the Galactic Guardians in the vintage toy line. But his figure got cancelled because Mattel couldn't get his action feature working, as such Darius wasn't in the list of characters presented to Jack Olesker and Jetlag to use in the NA cartoon. As a writer Jack saw the lack of a leader and mentor character and knew he'd need one so he created Sebrian and bumped Hydron up in rank in order to fill the roles left by that lack of Darius.
But by this point NA information and style guide based on the prototypes had already been sent out to other licensees which is why Darius, Crystoll and Detector appear in other NA products and media.
Here's his bio from the 1990 Annual, notice it's Darius and Icarius that travel to Eternia not Hydron and Icarius, because this is how it was originally intended to be, they only changed it to Hydron after Darius's figure had been cancelled.
Darius needs to be in Classics just as much as Flogg does!