WayfinderTerra is the first of Aqua's close friends. They were both pupils of Eraqus and had the same dream of becoming a Keyblade Master. Though not much of their past together is shown, I can easily see them training together and working hard to help each other accomplish their shared goal. I believe that they both think very highly of each other's skills and consider themselves to be evenly matched. When the Mark of Mastery exam goes haywire due to Xehanort's interference, I somehow got the impression that the two trusted that the other had their back while fighting. Jealously, however, is something that probably cannot realistically be avoided when two people have the same goal. While Terra and Aqua were taking the exam and fighting against each other, Terra got the sense that Aqua's skills were slightly above his own. He knew there was no winner or loser. Even so, his heart faltered for a minute and darkness fueled his desire to beat Aqua rather than to accept her skills being at a higher level. When Aqua was the only one of them to pass the exam, the psychological wedge in their relationship only got bigger. Though Terra does not say anything directly, I think Aqua could tell that he was clearly upset and could feel the distance growing between them.
As time passes, their relationship only becomes increasingly stormy. Aqua emotionally betrays Terra's trust by not only keeping tabs on him for Eraqus, but also because she accuses him of not following the right path. After seeing his reaction to the truth, Aqua clearly feels awful. There is no doubt that she treasures her relationship with Terra. The last thing she would ever want to do is make him feel cornered or alienated. Unfortunately, she had a moment of weakness and did just that. Seeing that her weakness sent him over the deep end is probably something that stays in the back of her mind as she continues to lose him more and more to the darkness. Though the game never makes any mention of her thoughts after the fact, I would not doubt that she feels somewhat responsible for not being supportive and caring like she should have as his friend.
Even so, Aqua never stops believing in Terra. From the moment she leaves The Land of Departure until the very end, she firmly says multiple times that she believes he will not give into the darkness. She believes in his strength, motivation and compassion. She refuses to believe Maleficent when she is told that Terra aided in stealing Aurora's heart. She refuses to believe Hades that Terra asked for advice on how to use the darkness. She absolutely cannot believe Yen Sid when she is told that Terra had a hand in Eraqus' death. Even when Xehanort completely has control of Terra's body, she still calls out to him, believing that he can regain control of his consciousness once again. She sacrifices herself in order to send his body back to the Realm of Light, not only simply because she wanted to save her friend but also because she believed that Terra would win the fight for himself when he reached the surface. Aqua's trust in Terra is one of the hallmarks of their relationship. He may not see it, but as a player of the game, I can definitely feel it.
I've thrown around the idea about Aqua and Terra possibly being more than just friends for a few months now. The game gives no indication that the two have any romantic interest in each other, but I can't say that I would be upset if future games hinted at Aqua having romantic feelings for Terra. Aqua cares for him a lot. Though she worries about Ven during her journey, she is almost always thinking and worrying about Terra. When she takes his hand to put Stormfall in it and send him back to the Realm of Light, I was just struck by how much love she felt for him. Again, not in a romantic sense, but I could tell how much she just wanted to make sure he would be okay. There are very strong feelings there that could be a base for something more. On the other hand, they may have been together for so long under Eraqus that they are more like family than romantic partners. Unfortunately we have yet to have any kind of window into their past before Ven arrived so I cannot say for sure. Regardless of whether or not romance could blossom between them, what really matters is that the two have a solid relationship foundation. Despite the turbulence of events around them and their relationship being put to the test because of it, they still managed to remain close friends that care about each other greatly.
Aqua's second close friend is Ventus. Despite considering him her friend, Aqua ends up treating him more like a little brother than anything else for a good portion of the game. I personally believe their relationship started off that way because he was unconscious for a long time when he first arrived under the care of Eraqus. Aqua stayed by his side taking care of him until he woke up. For her, it is probably hard to erase the image of Ven being someone that needs to be cared for out of her mind because that is how they were introduced. Whereas she feels that Terra would have her back if there was some kind of danger, Aqua would make it her priority to make sure Ven was not in danger rather than know he, too, had her back. Regardless, Aqua treasures her relationship with Ven as much as she does her relationship with Terra. She would still do anything she could to make him happy and ensure that he was out of harm's reach. She still worries about him after he leaves the Land of Departure suddenly and wants to defeat Vanitas in order to know that he would not be able to hurt Ven anymore.
As I have already mentioned briefly while talking about Aqua's various roles, treating Ven more like a child than an equal causes their relationship to build tension as well. He wants her to see him like she sees Terra, not as someone who needs to be taken care of all the time. Aqua has a difficult time breaking her old mentality, but she does eventually understand that Ven is not a child and is one of her precious friends just like Terra. After Ven loses his heart, she carries him on her back to find a place for him to stay while she takes care of things. Ven somehow opens a portal to the Land of Departure and Aqua smiles and agrees as long as that is where he wants to go. I think going with his opinion rather than trying to do what she thinks is best for him really shows that she believes in him as his friend.
Aqua's only regret as she fades into the Realm of Darkness is that she will not be able to make it back to Ven to wake him as soon as she promised. Clearly if that was the last thing she thought about before "disappearing" (since she did not know what would happen to her), he is just as important to her as the person she sacrificed herself to save. Ven has a precious place in her heart alongside Terra.
There is no question that Terra and Ven are the most significant people in Aqua's life. She makes the Wayfinders for the three of them in order to ensure they would always be connected no matter what happened. Her motivation to continue forward during her journey is all for them. While she gives them strength by supporting and connecting them, Terra and Ven also fuel Aqua's strength on numerous occasions. When Aqua's own power is faltering such as during her final battle with Vanitas, she holds her Wayfinder and asks for them to lend her strength. She feels re-energized and is able to defeat him. Any time she is feeling unsure about anything, she always takes her Wayfinder out for comfort. The most important instance of Terra and Ven giving her power is when she wanders around in the Realm of Darkness and becomes completely surrounded by giant shadows. She stands to fight but hesitates shortly after. What was the point in fighting? Maybe she should just fade into the darkness there. At that moment, Terra and Ven's Keyblades come from above and destroy all the shadows around her. Aqua is shocked but then remembers all of the people who love and care about her and all of the friends she made along her journey. She takes out her Wayfinder and realizes that she can't give up yet. Terra and Ven haven't given up on her just like she never gave up on them. Despite the outlook being bleak, Aqua continues forward for many years to come knowing that her friends were out there somewhere and that somehow the three of them would be reunited. Until that day, she could not allow herself to give up.