https://www.gamespot.com/articles/assassins-creed-odyssey-easter-egg-lets-you-unlock/1100-6462289/

Assassin's Creed Odyssey takes place before all of the other Assassin's Creed games, including (oddly) the one called Origins. Still, it pays homage with a few winks and nods, and one particular cameo comes straight out of an Assassin's Creed game separated by a couple of millennia.

As noticed by JorRaptor on the Assassin's Creed subreddit, you can unlock Evie Frye as a lieutenant on your ship, the Adrestia. She provides a bonus to javelin and fire javelin damage as well as provide a damage boost while your health is low. She can be unlocked in the Ubisoft Club after completing the "Master Assassin" Challenge. That reward comes for reaching 7,500 XP in all Assassin's Creed games. That essentially rewards you for having played a prior game, since it will likely be unlocked from the start.