Coincidence?

        Harry Potter. Percy Jackson. It seems like you can't get enough of either. They're both so unique, both so compelling. So why in the world is there so many similarities?
        I mean, there are obvious ones. Let's start with the fact that both of our main characters, Harry and Percy, have the exact same features. Apart from the scar and glasses, both characters have jet black hair and bright green eyes. Both of them look surprisingly like their fathers, except for a few key features (Harry's eyes, Percy's attitude.) 
        Besides the physical features, both have come from rather abusive backgrounds, and neither had known their parent(s) until a well-enough age (twelve for Percy, never for Harry.) Both are better at practical fighting/dueling, whereas they, *ahem* lack in the brains department.
        Then there are Harry and Percy's friends. Ron and Grover are the comic relief, the characters that have no major role, yet are necessary for the story. They are our supports, the figures that keep our main characters going. Another minor detail is their avid love for food. 
        Hermione and Annabeth are both the smart thinkers of our trio. I wouldn't be surprised if Hermione turned out to be a daughter of Athena in the Percy Jackson universe (if that happened, then all of Hecate's kids would be Hogwarts, or rather Ilvermorny, students.)
        It is also fit to say that the figures Chiron and Albus Dumbledore portray are very similar, in the way that they are both well-liked and appreciated mentors. Many of the heroic deeds accomplished would have been downright impossible without either characters help. 
        Lots of other characters have doubles in both universes: Luke and Draco, Clarisse and Snape, Hagrid and Tyson, Rachel and Professor Trelawney, Sally Jackson and Sirius Black, Fluffy and Cerberus (the fact that Hagrid got Fluffy from a "Greek chappie" says a lot), the Stoll and Weasley twins; the list goes on. 
        Then, there is the words themselves. The term 'half-blood' is very common in both sagas, though the meanings do vary. Wizarding spells are derived from Latin (lumos, the spell for light, is derived from lumen, Latin for light), and Ancient Greek weapons all have Greek names (Anaklusmos, aka Percy's sword Riptide).