^Eh I wouldn't go that far. Ven is closer to Sora in personality but later on becomes more like Roxas. Aqua is only like Sora in the sense that she can be argue as THE main character. --- "There is nothing either good or evil, but thinking makes it so"
Okay, I'll concede the Sora/Aqua point, but on the other hand...
*warning: large spoilers*
Sora and Riku were friends before Kairi suddenly appeared one day; Aqua and Terra were friends (or possibly just fellow students/apprentices, I'm not entirely sure) before Ven suddenly appeared one day.
Kairi is the weakest of her friends by dint of only fighting in one cutscene in the entire series (thus far); Ven is the weakest of his friends by dint of Terra and Aqua specializing in strength and magic, while he specializes in speed (thus his individual hits would not be as strong as Terra's or Aqua's).
Maleficent needed Kairi's heart to complete the final Keyhole so she could open the Door to Darkness, or some such nonsense; MX needed Ven's heart to create some ridiculous sword which apparently does...something...to Kingdom Hearts.
Kairi loses her heart, which resides within Sora's body until it is returned; guess what happens to Ven?
...Okay, I marked the spoilers clearly, so I should be fine, right? And I think I had another point earlier, but I can't remember it now. Oh, well.
I would say Aqua is the most important. The entire KH series pretty much stems from her actions. --- Formerly known as Darkmagic3 The quickest way to a girl's bed is through her parents, have sex with them and you're in!-Zapp
Terra plays the main character as everything in the story does really revolve around him. Personality wise he's the riku of the group and their roles are so similar it's almost scary.
Aqua really DOES play the hero and in actions and role she is the Sora of the group, the one that plays the hero of the whole story.
Ven honestly is the kairi of the group (in a LOT of ways..you could say they even have similar hearts really >_<). He plays the role of the tag along who is used constantly to make main character angry and fall deeper into the villain's plan. He was the one who was used more as an object to obtain what was desired than the value of a person himself.
Plot wise thought it doesn't matter where you play Terra tbqh. But Aqua's story spoils basically the ultimate plot point of Ven's story and so you really don't want to spoil that so I suggest you play Ven and then Aqua.
..on a side note I just realised the Sora would likely have played the hero and Riku the main character if Sora hadn't stolen the keyblade from Riku in the first place and so in a way Sora stole Riku's show away from him >_> (ultimately that saved Riku from a fate similar to Terra's but still..) --- Official Roxas of the Kingdom Hearts: Birth By Sleep Board ^_^
Personality wise he's the riku of the group and their roles are so similar it's almost scary.
Mm, you could make a case in terms of role, but personality-wise, Terra isn't really like Riku at all. Aqua also isn't very much like Sora, or Ven like Kairi, which is nice, since it mixes things up a little.
Eh not really Riku and Terra personality wise and very close to the same. Riku just had a different teacher. As Aqua said Riku was so pure and innocent just like Terra. But Riku's teacher was the evil AnsemSoD and Maleficent while Terra's was Eraqus and Master Xehanort.
The teacher in this case is the thing that changed their personalities so greatly. Oh yeah and the whole age difference as well. Riku was a teenager so he still had kid like personalities while Terra is a full grown man in his youth. Riku became a lot more like Terra as he got older actually just Riku lost the ego as a kid and Terra didn't loose it till it was to late. --- Official Roxas of the Kingdom Hearts: Birth By Sleep Board ^_^