We virtually know who they are, but I figure this is more for...figuring out parts they play.. And how relationships they will have..Also for deciphering the Prophecy.
Also, For the Mark of Athena..I think the Mark of Athena will be from Annabeth's point of view, or her and maybe Jason and Percy. Or something. Her, Hazel and Piper? Maybe it'll be just Jason, Leo, and Piper's? and Book four will be from Perc's and Hazel's and Franks. But maybe since Son of Neptune was four chapters ( i think..) with each person, One for all seven? I think it'll be from hers, and most likely Jason and Percy's.
Frank and Leo, have a lot in Common. Both of their moms died, and both their dads are disliked by Hera! But! Leo has ability over fire, and Frank's life force is tied to a log, his gifts apparently are too powerful for himself. Fire, and Logs.
Anyhow, Jason and Percy may clash or may work well together. They are were praetors, respectively.
Then, most likely Hazel will get all nervous around Leo, and rain gold and gems, owing to the "Sammy Valdez" Fact. (I think Sammy was just Leo's mom's grandpa or dad. But is anything that simple in this series!?!?!?!)
Annabeth, is gonna be a hinderance, but if she's one of the seven, she's gonna hav'da come. Annabeth for one, HATES Hera. She'll hate her more for switching Percy. Probably.
Leo is, as Hera puts it, her Champion. He's a awesome pro. The Prophecy says "To Fire or Storm the World must fall." I have a theory about that, I'll tell you later. He'll play a uberpart, and without him, the Argo II won't run. It probably has machine parts, so he'll be MORE important, to make him go! (it's a him cause Festus is a Him, Right? Do machines have genders? Nevermind. He'll make it go whatever)
Frank is Hera's, Secret weapon. A deadly archer, of both Greek, Chinease, and Roman Blood (imagine!) His gifts are so awesome, that he has to have a weakness. A.k.a the log. Also, I bet the reason Gaea thinks she can maniplulate Percy, is because of Perc's flaw, and like Mars said, Frank will be able to keep them going through his duty. That way, it'll be harder for Gaea to maniplutate Percy. He's Juno's weapon, so if he can keep them going, and keep Percy from being manipulated (to a degree..), Percy can be the "Glue", GET IT??!!?!?
Annabeth. I always liked her, but if she gets them in trouble, like we know she will... Anyhow, with her war knowledge and stuff, she's gonna be important, how, I dunno.
Hazel, is gonna be awesome. She's gonna face hard stuff, but her powers will kick in. Basically, I dunno what she will can do.
Piper! She can Charmspeak her way to people! Though I doubt it'll work on Annabeth. She's gonna be so much Friggin help.
Jason... I'm just gonna save this for now.
I'll edit later... *puff*