To make something clear in these posts, I want to say that although we constantly complain about how gullible Terra is, that's not inherently a flaw with either his character or the story. Gullibility is a trait, like any other. It can be good, bad, or somewhere in between.
The real problem comes from the fact that Terra is fooled by the bad guy every single time. No matter how many times he is ruthlessly, consistently fooled, he never takes steps to try to avoid being duped in the future. It's always "Well, I guess I've been duped again. Guess I couldn't do anything about that".
And even that could work, but then it would help if he had some character trait other than "stupid, easy to trick". Just some scenarios where his gullibility generates some interesting dynamics b/t him and more heroic character, or if he grew wise and started raising questions to certain people before he chose to help them. The latter could even mark some strong, much needed character growth.
Instead, the same scenario plays out over and over again. It gets to the point where one can easily predict the plot of almost every world.
- Terra will find himself fooled by the bad guys, up until he helps them complete their plans. At this point, once they think they no longer need him, they will double cross him, leading him to realize he'd been duped.
- Ventus (Call him, Ven. will show up either slightly earlier/later, and just miss him. While looking for Terra, Ven meets the friendly Disney hero of whatever world he landed on and get sidetracked into helping him for a bit. Once he feels he can do no more, he'll leave while the Disney hero takes care of it from there.
- But no, actually Aqua shows up after the other two to tie-up loose ends, learning that she "just missed" her friends.