Question: Was Odysseus a biological grandson of Zeus adopted as a Scion of Athena, a Scion of Zeus, or just a really clever mortal who lived a particularly legendary life surrounded by gods, demigods, and monsters?
Odysseus, in addition to being the Greek hero most disagreed about by scholars in terms of his purpose and philosophical representation, is also one of the messiest to try to pin down geneaologically. In all technicality, by Scion's rules, he couldn't have been a Scion as he's too far removed from his divine forbears, but he certainly has more divine heritage than most mortals could claim.
He's not Zeus' grandson, but depending on the mythic version could be considered either his great-grandson or great-great-grandson. According to Homer, he is the son of Laertes, who was himself the son of Arcesius, himself a son of Zeus, making him three generations removed; at the same time, his mother is Anticlea, who was the daughter of Autolycus, who was the son of Hermes (who is of course the son of Zeus), making him simultaneously Zeus' great-grandson, great-great-grandson and Hermes' great-grandson. And if that wasn't confusing enough, there's a variant myth in which his father is actually Sisyphus, which would again make him a great-grandson of Zeus through Sisyphus' father Aeolus, one of Zeus' many sons.
This is all a bit head-spinning, but what it basically boils down to is that, by Scion's rules, Odysseus could not be a Scion. He would need to be at least the grandson of a god, and he's one step too far away in all cases. This is also supported by the fact that, despite his excellent adventures and exploits, he eventually died of old age in Ithaca, a distinctly mortal kind of deceasement.
But Odysseus also does many great deeds, succeeds on many great quests and sometimes even out-performs dudes who definitely are Legendary; it's hard to believe that a mere mortal could have done all that, or that the gods should have bothered to take that much of an interest in him (or, come to that, that he could have survived Poseidon's snittiness for so long). So what to do with the most famous mind in ancient Greece?
The easiest option is probably just to declare that he was a Scion and insert some plot to explain whose and why his genealogy isn't preserved. Perhaps he was a closer relation of Zeus' or Hermes', hidden from the other gods as a secret weapon, or the son or grandson of some minor god taken under Athena's wing. There are pretty much unlimited ways to do this; the one that's closest to existing mythology would be to consider him the son of Sisyphus, and further to consider that his grandfather Aeolus was in fact identical to the weather-god Aeolus rather than a separate mortal king, allowing Odysseus to be the Scion of a minor god of the Dodekatheon who also happens to be related to Zeus and Hermes. If your plot requires Odysseus to be a Scion, though, you can pretty much assign him any secret parent you want, from a Zeus avoiding Hera's attention to a shocking child of virginal Athena herself. Of course, he will go off to die in peace at the end of his story, but consider that Odysseus, out of all Greek heroes, is the one guy who never wanted to be involved in epic things and just wanted to stay at home with his family. He might be the only example I know of in mythology of a Scion who chooses to allow himself to lose his Legend through inactivity, a possibility the books include but which I've never heard of anyone using in a game.
Of course, that's deciding to make up a new geneaology where normally we take the ancient Greeks and their myths at face value, and that's not our favorite idea. Another simple solution that might appeal is to set Odysseus as a Legendary Mortal; it would give him a little bit of a Legendary boost, certainly enough to remind us who he is and keep his legends and exploits strong, but would accept that his geneaology indicates he wouldn't be a full-blown Scion without fudging. Of course, how he came to be a Legendary mortal is really up to the Storyteller and the needs of the story; the Scion books are somewhat vague on this point. One of the most important themes of the stories of Odysseus is that he is a mortal; he's the example of mortal perfection and mortal excellence that the ancient Greeks so loved, and the fact that he can succeed alongside demigods is important partly because he isn't one of them. Setting him as a Legendary Mortal allows for brief flashes of his legendary awesomeness, but still retains that idea of him as a representative of the greatness of humanity.
Finally, you could look at the possibility that he was entirely mortal, but Fatebound to the major Legendary players in the Trojan war - probably at least Achilles, initially, but maybe even Athena and Poseidon themselves later. It's not very likely that he could hold his own as a mortal in such company, but if he had just the right Fatebound roles - a Boon Companion for Athena, perhaps, or a Rival against Poseidon - Fate itself might have helped him go far above and beyond what he normally could have done as a result. I'm actually not a big fan of this option only because it would suggest that Odysseus is famous because of being a character in the stories of others, and while he certainly isn't spotlighted for most of the Iliad, the following Odyssey is all him, and suggests to me that he was making a legend of his own, not merely acting as an adjunct to someone else's.
But, as you can see, there are plenty of options available. I think I might personally be fondest of the idea of Odysseus as a Legendary Mortal, myself, because I think it strikes a neat balance between staying true to mythology but allowing him to be awesome in his own right, but I think any of them might work in the right story.