are you kidding they look the same oh whatever it`s just a game I really don`t get some of the stuff in the games though like how they made birth by sleep and the days one after the main ones but the games are based way back before the first game ever started it really confuses me
The main reason why Roxas isn't related is because Ventus is a normal human and Roxas is Sora's Nobody. Another reason is because if you completed all the characters you get an extended Aqua story and it makes a cliff hanger. I can explain the rest of the extended story if you reply again.
Here's what I found on the wiki. I tried to word it but I just couldn't (I do remember this happening so if it sounds funny just believe it. It starts just before the end so, yea...)
Terra, Aqua, and Ventus gather at the Keyblade Graveyard, where Xehanort appears with Vanitas and reveals his plan to use the χ-blade to unlock Kingdom Hearts and start a Keyblade War, an apocalyptic event that nearly destroyed the world in the past. Terra's heart is swallowed by darkness in the ensuing battle, allowing Xehanort to take over his body, but Terra's mind remains within his discarded suit of armor, which comes to life and defeats XehanortMeanwhile, Ventus is forced to fuse with Vanitas to create the χ-blade and battles Aqua alongside Mickey, Yen Sid's apprentice, while he engages Vanitas in a metaphysical battle within his mind.
Aqua destroys the χ-blade as Ventus defeats Vanitas from within, though Ventus loses his heart and falls into a deep sleep as a result.
After learning of Ventus' state, Aqua brings his body to the Land of Departure, where she transforms it into Castle Oblivion to ensure Ventus will not be disturbed. She then goes to Radiant Garden where she fights Xehanort, who still possesses Terra's body. The battle rages until Xehanort unlocks his own heart after Terra's heart tries to regain control of his body, and falls into the Realm of Darkness. Aqua saves Terra, but becomes trapped in the Realm of Darkness in the process; Xehanort, who remains in Terra's body, loses his memories and is found by Ansem the Wise, ruler of Radiant Garden; and Ventus' heart reconnects to the heart of a young Sora where it is able to rest.
In the game's secret ending, which takes place after the events of Kingdom Hearts II, Aqua meets Ansem the Wise in the Realm of Darkness and learns from him about Sora, who has the power to save those connected to his heart, including Terra, Ventus, and Aqua herself. Sora, having learned of their fates from King Mickey's letter, undertakes another quest to save them.