Note: this is my theory, demean me, argue with it if you want,ect it's your choice.
Okay, what we know so far (or at least I know so far) is this.
Master Eraqus: sensei/master of Aqua, Terra, and Ven/Ventus, friends/acquaintances with Master Xehanort, able to use, and summon a Keyblade,(maybe...