Terra becomes the first heartless after the final boss. The whole him being Xehanort is a little too obvious to be true.
Aqua gets killed by Vanitas (Final Boss). She has the least connection to the original story line.
Ven gets erased but his memories arent. The memories search for the closest home and find sora. This gives Roxas the appearance of Ven.
MX and Erauqs get in a fight and both go missing until KH3.
Vanitas is the final boss. TAV cant beat him but they can erase his memories. He turns up after the battle in Radiant Garden and all he can remember is his masters name and so he takes it for his own.
Erauqs is the same as MX