I'll bite, these characters are a lot more fleshed out than you are giving them credit for. Using Xehanort's quote about them gives them a better light regarding that:
The broken boy, Ventus having been torn apart by Xehanort's ambition and trying to make something of himself despite the fact that he didn't know who or what he was. Especially now coming from the Age of Fairy tales he seemed to have lost his memories of everything that happened. He potentially was brought to Xehanort by Luxu considering that Luxu had the No Name keyblade.
The feckless youth, Terra was nearly responsible for every negative thing that happened in every world because he irresponsibly listened to every villain and didn't realize what he did until it was too late. The darkness inside grew and grew because of his desire to prove himself and then he had to strike down his own master to protect his friend and after all of that he lost his battle with Xehanort and had his body taken. Also, without him Riku would never have gotten a keyblade and in turn Sora.
The misguided master, Aqua could have been given better advice from Eraqus but because of her master's severe hatred of all things darkness she went in with a similar mentality and put duty over friendship without realizing that her friends were struggling and because of that she shouldered all of their problems on her own. Her sacrifice is pretty much what made Kingdom Hearts what it is now, you could blame her for everything. She turned Land of Departure into Castle Oblivion and hid Ventus, giving them reason to put the Organization there (and potentially setting up another plot with Marluxia/Lauriam and Larxene/Elrena). She pulled Terra-nort out of the darkness and let him continue his life at Radiant Garden which in turn was the reason it was swallowed by darkness and lead to the adventures that happened there, indirectly making her the cause of Ansem and Xemnas' existence, and she's been fighting as hard as she can, as long as she can muster, for ten plus years trapped in the realm of darkness which any weaker heart would have succumbed long ago.
They are all tragic but they are all a major part of the story at this point.