I was thinkin maybe Pre-deaths. All who lost there lives in regret and unfullfillment. they feed on the energy of anothers soul (producing more Pre-deaths kinda like a vampire) searching for their missing pieces in their journey to become whole again. only once it has reunited with its unbirth nobody and heartless will it reform to its original form. and the symbol would be like well the basic idea is a COM card w/ half an X in it and half a heart (connected to card) w/ the other half of X in it. (basic idea)(feel free to draw it and post) (forgot what I was gunna say next....Oh well.)
My 3 favorite people are Riku,because he walks the path of twilight, and Roxas because he's just awesome. and Axel (got it memorized?...Good. your a quick learner)
my theories for ven/roxas are still in progress,but i came up with a few ideas.
one of them is that Ven's heart is lost,but is dorment inside of sora(like Kairi's from kh) because he was destined to be a keyblade master. from what i heard Ven has a strong heart . when it was released from sora it assumed the form of roxas and since vens heart was dormant in sora for a few years he is remembering sora's memories. (and similar fighting style) still roxas was sora's nobody but since ven had a strong heart and wielded the keyblade Roxas was able to wield a keyblade and was different from other nobodies. (the rest is still in progress but so far its just a guess)
PS thank Renxssa's comment (it gave me an idea for my theories)
I would probably play as Axel Marluxia Roxas Saix (not precisely that order) and (if) anyone that's quick ( i like using quickness to get through strength)
When he is using sleights or cards 7-9 use a 0 card to break it.
when he uses lightning waves jump over them or card break.
if you run out of 0 cards reload. (not too much)
if you get too low health use the Oogie Boogie card (I think) it slowly heals you
to avoid some attacks in dark form just double jump to get behind then attack
when you use mickey cards use them in sleights (if all low # use attack card last
Also you may need to raise your lv (hp/ap/dp)