[Kaos2005]

Originally Posted by Kazu-kun

No it's pretty clear they're not. Kouga and Aria on the other hand, there's a ton of foreshadowing hinting they're related.

My wild theory is that Kouga is related to...

Spoiler:

Eden and Sonia and he's actually another son of Mars.

That would explain the dark crystal...

But also, yesterday i was re-watching the first episode and near the beginning, when Seiya fights Mars, baby Kouga said "pa" (maybe from "papa"?) while looking at him and he was also like trying to reach him with his arm... it also could be directed to Seiya, but there is no point in not telling him that Seiya is his father and that also would not explain the light/dark crystal/cosmo.