Terra makes many friends and enemies on his travels throughout the worlds. Like with most keyblade wielders it is his relationships that save him. I feel that although Terra tried to do much himself, it was only because he relied on a very specific set of people to cover for him. In particular, if anything were to go wrong for any reason, he trusted and knew that Aqua and Ven would be able to take care of it.
Eraqus
Terra's relationship with Eraqus is very father and son. Both see each other in this light. I tend to think that Eraqus was a very strict master yet a very loving (maybe even doting) father. I feel like he always kept the light close at heart and that became the focus point of all his lessons. I believe that he probably even had difficulty separating the two jobs of master and father, and sometimes forgot that one role was more necessary than others at times. Eraqus has high expectations of Terra. It is his love for Terra that allows him to give Terra a second chance. It is also his love for his children that drive him to attempt to stop Xehanort by trying to kill Ventus. He wants to protect his children. After his defeat at Terra's hand, he admits how wrong he was. His death results in Terra wanting to take revenge for being manipulated.
Ventus
While Ventus and Terra are also rivals in their goals of becoming keyblade masters, they share a close brotherly bond. Aqua comments that they make strange brothers, but I feel like she's actually quite happy that they look after each other. In fact, when things get shaky for Ventus, he runs to Terra. I think part of this is becuase Terra was the one that opened up the dream of being a keyblade master to Ven. When Ventus first joined them, Terra was naturally curious and asked him too many questions that the boy could not answer. This sent Ven into shock. Believing that he had injured him in some way, Terra decided to look after him and began treating him as a little brother. In addition, when Terra and Aqua are sent to check out the unversed, Ven wants to go along. Terra thinking Ven is just being clingy (and probably a little depressed himself) tells Ven to stay behind. Ven leaves after Terra anyways and is usually the first one to jump to Terra's defense. He stands up to Maleficient when she says that Terra steals Aurora's heart, he stands up to Aqua when she wonders what Terra has been up too, and he gives up one of his most prized treasures to the treasure chest of the lost boys - an old training keyblade with Terra's name on it (probably the first gift he ever received).
Aqua
Aqua and Terra strike me as brother and sister, I feel like they grew up together and did everything together. This makes it easy to share responsibility, friendships, and dreams. Their dynamic is wonderful and I like that despite any arguments they may be having they always agree on Ventus. Aqua becomes a keyblade master in the Mark of Mastery, she looks almost as shocked as Terra does when he does not pass. I think she felt partly to blame (for whatever reason) and was saddened that they did not achieve their dream together. She is given knowledge that Terra is not when they have to go find the unversed. She is put in the very awkward position of watching Terra, despite the fact that she initially believes he does not need it. As she travels through more worlds, she sees a few acts of darkness that have been attributed to Terra and begins to question and doubt him. After Eraqus is killed she believes that Terra's emotions are causing the darkness within him to grow and worries that his quest for revenge will lead him onto a path that no one can save him from. A fair point, I believe because she knew him so well. When she sees him fighting Xehanort with everything he has she understands his pain. But it is his keyblade's movement after she falls into the Dark Meridian that ultimately reassure her that Terra had only ever been trying to protect her and Ventus.
Xehanort
Xehanort is probably my least favorite character in existence. The more I understand about him, the more I want to scream. It is difficult for me to describe how much I want to punch him in the face (which is now Terra's sweet face). Xehanort is the third keyblade master of Eraqus's generation (the other being Yen Sid). He appears during the Mark of Mastery to check up on Eraqus and his pupils. Under the guide of curiosity, he plants the seed of Darkness within Terra and slowly begins to corrupt him. Through gentle prodding and listening, Xehanort is able to convince Terra that darkness (as power) is not all evil and that Eraqus is blinded by light. Through his interactions he is able to help the darkness within Terra grow. This leads to betrayal as Terra fights Eraqus to save Ventus and watches as his master is killed. Ultimately, the understanding that Terra thought he got from Xehanort was nothing more than a ruse and was instead a way for Xehanort to overcome Terra and take his younger body. Terra's will is so strong he summons moves his armor and keyblade with the strength of his heart, but is unsuccessful in expelling Xehanort from his body.
Riku
Riku is a little boy that Terra chose as his successor. I have mixed feelings about that, which I will detail in another section of the site. Terra chooses Riku as his successor because he sees in Riku the best traits he sees in himself. He understands Riku's desire to protect his friends above all else. Despite his growing doubt, that was the one thing Terra tried to keep closest to his heart. Although their interaction as brief, Terra's need to choose a successor also strikes me as bittersweet.