Kingdom Hearts Relationships
Below you will find information detailing Aerith's relationships with the other characters of Kingdom Hearts. Please note that the Kingdom Hearts series is considered an 'alternative universe', meaning any events or interactions between characters only apply within the series itself.
Leon | Yuffie | Cloud | Sora, Donald & Goofy | Cid

Leon grew up with Aerith in Hollow Bastion, also known as The Radiant Garden, along with Yuffie and Cid. The four of them travelled to Traverse Town aboard a Gummi Ship after the Heartless took over their home world and practically destroyed it. Leon, Aerith and the others began to research the Heartless in the hope of finding a way to defeat them, using the scattered reports left behind by Ansem the Wise. After aiding Sora, in his quest to rid the worlds of darkness, they are able to return to Hollow Bastion. They then set up the 'Hollow Bastion Restoration Committee' in order to restore the town to its past splendour.
Aerith and Leon have a close relationship in Kingdom Hearts. Aerith is able to read his emotions, and interpret them for others, while Leon seems at ease in her company. Aerith and Leon share the same wisdom when it comes to the Heartless, and it is likely that most of the information the group have on the subject is down to their combined efforts.

Yuffie grew up in Hollow Bastion with Aerith and the other Final Fantasy characters, and travelled with them to Traverse Town after the appearance of the Heartless forced them to leave. She then began helping Aerith and Leon to uncover more information on the heartless and the 'key'. Later, she returns to Hollow Bastion with the others to begin the restoration project.
Aerith, being the older and more mature of the two, looks out for Yuffie and acts like an older sister to her. However, with Yuffie Aerith is also able to express her sense of humour and often joins her in poking fun at overly serious Leon.

Cloud, like Aerith, comes from Hollow Bastion but he did not join the others when they fled to Traverse Town. Instead he left on a quest to confront his inner darkness and find his light, both of which manifest in the form of Sephiroth and Tifa. The brief meetings that Aerith and Cloud share hint at a special relationship but not necessarily a romantic one. Cloud distances himself from Aerith, as he focuses on his quest which he does not want her involved in. It is clear, that despite his coldness, Aerith cares for him very much and worries about his well being.

Sora, Donald and Goofy meet Aerith on their first visit to Traverse Town, albeit at different times. Donald and Goofy are discovered by Aerith wandering the streets, who guesses they have something to do with King Mickey. She then tells them everything she knows about the Heartless, other worlds and the mysterious 'key'. Aerith meets Sora, who has been getting a similar lecture from Leon, shortly after. Aerith then helps the three heroes throughout their quest by sharing her wisdom and providing them with useful items, such as the missing Ansem Reports.
Aerith also makes a notable appearance in Kingdom Hearts: Chain of Memories, as a fragment of Sora's memory in the Castle Oblivion. Her wisdom intact, she warns Sora not to be deceived by the illusions he may see in his journey through the castle. A lesson he does not fully take on board.

Cid and Aerith both come from Hollow Bastion, and travelled to Traverse Town in a Gummi Ship with Yuffie and Leon when the heartless began to multiply. In the new world they appear to go their separate ways, Cid runs the local item shop while Aerith and the others search for information on the darkness. When the group later return to Hollow Bastion, both Aerith and Cid are members of the 'Hollow Bastion Restoration Committee'.
Aerith and Cid have not had any interaction in the series so far, but it is fairly safe to say they are on good terms due to the fact they live and work together, as they attempt to rebuild their home town.